MLL5 (KMT2E): structure,function, and clinical relevance

The mixed lineage leukemia (MLL) family of genes, also known as the lysine N-methyltransferase 2 (KMT2) family, are homologous to the evolutionarily conserved trithorax group that plays critical roles in the regulation of homeotic gene (HOX) expression and embryonic development. MLL5, assigned as KMT2E on the basis of its SET domain homology, was initially categorized under MLL (KMT2) family together with other six SET methyltransferase domain proteins (KMT2A–2D and 2F–2G). However, emerging evidence suggests that MLL5 is distinct from the other MLL (KMT2) family members, and the protein it encodes appears to lack intrinsic histone methyltransferase (HMT) activity towards histone substrates. MLL5 has been reported to play key roles in diverse biological processes, including cell cycle progression, genomic stability maintenance, adult hematopoiesis, and spermatogenesis. Recent studies of MLL5 variants and isoforms and putative MLL5 homologs in other species have enriched our understanding of the role of MLL5 in gene expression regulation, although the mechanism of action and physiological function of MLL5 remains poorly understood. In this review, we summarize recent research characterizing the structural features and biological roles of MLL5, and we highlight the potential implications of MLL5 dysfunction in human disease.     

Low-density lipoprotein receptor structure and folding

The endoplasmic reticulum (ER) is a major cellular 'production factory' for many membrane and soluble proteins. A quality control system ensures that only correctly folded and assembled proteins leave the compartment. The low-density lipoprotein receptor (LDLR) is the prototype of a large family of structurally homologous cell surface receptors, which fold in the ER and function as endocytic and signaling receptors in a wide variety of cellular processes. Patients with familial hypercholesterolemia carry single or multiple mutations in their LDLR, which leads to malfunction of the protein, in most patients through misfolding of the receptor. As a result, clearance of cholesterol-rich LDL particles from the circulation decreases, and the elevated blood cholesterol levels cause early onset of atherosclerosis and an increased risk of cardiac disease in these patients. In this review, we will elaborate on the structural aspects of the LDLR and its folding pathway and compare it to other LDLR family members.     

Roles for CCN2 in normal physiological processes

CCN2, also known as connective tissue growth factor, is a member of the CCN (CCN1–6) family of modular matricellular proteins. Analysis of CCN2 function in vivo has focused primarily on its key role as a mediator of excess ECM synthesis in multiple fibrotic diseases. However, CCN2 and related family members are widely expressed during development. Recent studies using new genetic models are revealing that CCN2 has essential roles in the development of many tissues. This review focuses on current and emerging data on CCN2 and its functions in chondrogenesis and angiogenesis, and on new studies showing that CCN2 has essential functions during embryonic and postnatal development in a number of epithelial tissues.     

Annexins: what are they good for?

Annexins comprise a unique family of calcium- and phospholipid-binding proteins. At least one of the twenty members thus far described from this family can be found expressed in nearly every eukaryotic cell type. As common as these proteins may be, no one clear function for all has been established. Historically, individual members of this family have been given various names describing their ability to associate with a host of intra- and extracellular proteins and with cellular lipid membranes. The collection of reviews in this issue of CMLS represents an effort to offer a coordinated view of the research activities in the field and to extract structural and functional commonalities.     

Chimeras of human lysozyme and α-lactalbumin: an interesting tool for studying partially folded states during protein folding

Protein folding is an extremely active field of research where biology, chemistry, computer science and physics meet. Although the study of protein-folding intermediates in general and equilibrium intermediates in particular has grown considerably in recent years, many questions regarding the conformational state and the structural features of the various partially folded intermediate states remain unanswered. Performing kinetic measurements on proteins that have had their structures modified by site-directed mutagenesis, the so-called protein-engineering method, is an obvious way to gain fine structural information. In the present review, this method has been applied to a variety of proteins belonging to the lysozyme/α-lactalbumin family. Besides recombinants obtained by point mutations of individual critical residues, chimeric proteins in which whole structural elements (10 – 25 residues) from α-lactalbumin were inserted into a human lysozyme matrix are examined. The conformational properties of the equilibrium intermediate states are discussed together with the structural characterization of the partially unfolded states encountered in the kinetic folding pathway. Received 28 May 1998; received after revision 6 July 1998; accepted 6 July 1998     

Mechanisms controlling cellular suicide: role of Bcl-2 and caspases

Apoptosis is an essential and highly conserved mode of cell death that is important for normal development, host defense and suppression of oncogenesis. Faulty regulation of apoptosis has been implicated in degenerative conditions, vascular diseases, AIDS and cancer. Among the numerous proteins and genes involved, members of the Bcl-2 family play a central role to inhibit or promote apoptosis. In this article, we present up-to-date information and recent discoveries regarding biochemical functions of Bcl-2 family proteins, positive and negative interactions between these proteins, and their modification and regulation by either proteolytic cleavage or by cytosolic kinases, such as Raf-1 and stress-activated protein kinases. We have critically reviewed the functional role of caspases and the consequences of cleaving key substrates, including lamins, poly(ADP ribose) polymerase and the Rb protein. In addition, we have presented the latest Fas-induced signalling mechanism as a model for receptor-linked caspase regulation. Finally, the structural and functional interactions of Ced-4 and its partial mam malian homologue, apoptosis protease activating factor-1 (Apaf-1), are presented in a model which includes other Apafs. This model culminates in a caspase/Apaf regulatory cascade to activate the executioners of programmed cell death following cytochrome c release from the mitochondria of mammalian cells. The importance of these pathways in the treatment of disease is highly dependent on further characterization of genes and other regulatory molecules in mammals. Received 18 February 1998; accepted February 1998     

Tenascins,a growing family of extracellular matrix proteins

The tenascins are a family of large multimeric extracellular matrix proteins consisting of repeated structural modules including heptad repeats, epidermal growth factor (EGF)-like repeats, fibronectin type III repeats, and a globular domain shared with the fibrinogens. The tenascins are believed to be involved in the morphogenesis of many organs and tissues. To date three members of the tenascin family have been described, tenascin-C, tenascin-R, and tenascin-X. Tenascin-R seems to be specific for the central and peripheral nervous system, tenascin-X is most prominent in skeletal and heart muscle, while tenascin-C is present in a large number of developing tissues including the nervous system, but is absent in skeletal and heart muscles. Tenascin-C was the original tenascin discovered, partly because of its overexpression in tumors. Inferring from cell biological studies, it has been proposed that tenascin-C is an adhesion-modulating protein.     

Structure and function of desmosomal proteins and their role in development and disease

Desmosomes represent major intercellular adhesive junctions at basolateral membranes of epithelial cells and in other tissues. They mediate direct cell-cell contacts and provide anchorage sites for intermediate filaments important for the maintenance of tissue architecture. There is increasing evidence now that desmosomes in addition to a simple structural function have new roles in tissue morphogenesis and differentiation. Transmembrane glycoproteins of the cadherin superfamily of Ca²⁺-dependent cell-cell adhesion molecules which mediate direct intercellular interactions in desmosomes appear to be of central importance in this respect. The complex network of proteins forming the desmosomal plaque associated with the cytoplasmic domain of the desmosomal cadherins, however, is also involved in junction assembly and regulation of adhesive strength. This re-view summarizes the structural features of these desmosomal proteins, their function during desmosome assembly and maintenance, and their role in development and disease.Received 5 February 2003; received after revision 14 March 2003; accepted 1 April 2003     

The role of thrombospondin-1 in apoptosis

The thrombospondins are a family of extracellular proteins that participate in cell-to-cell and cell-to-matrix communication. They regulate cellular phenotype during tissue genesis and repair. Five family members, each representing a separate gene product, probably exist in most vertebrate species. Like most extracellular proteins, the thrombospondins are composed of several structural domains that are responsible for the numerous biological functions that have been described for this protein family. Considerable progress has been made towards understanding the function of thrombospondins. The role of thrombospondin in the process of apoptosis or programmed cell death has recently come into focus. In this review we will concentrate on the role of thrombospondin-1 in the broad field of apoptotis research. Received 5 December 2001; received after revision 28 March 2002; accepted 28 March 2002     

The cystic fibrosis transmembrane conductance regulator (CFTR) and its stability

The cystic fibrosis transmembrane conductance regulator (CFTR) is responsible for the disease cystic fibrosis (CF). It is a membrane protein belonging to the ABC transporter family functioning as a chloride/anion channel in epithelial cells around the body. There are over 1500 mutations that have been characterised as CF-causing; the most common of these, accounting for ~70 % of CF cases, is the deletion of a phenylalanine at position 508. This leads to instability of the nascent protein and the modified structure is recognised and then degraded by the ER quality control mechanism. However, even pharmacologically ‘rescued’ F508del CFTR displays instability at the cell’s surface, losing its channel function rapidly and it is rapidly removed from the plasma membrane for lysosomal degradation. This review will, therefore, explore the link between stability and structure/function relationships of membrane proteins and CFTR in particular and how approaches to study CFTR structure depend on its stability. We will also review the application of a fluorescence labelling method for the assessment of the thermostability and the tertiary structure of CFTR.     

Biochemical,biophysical and molecular dynamics studies on the proteoglycan-like domain of carbonic anhydrase IX

Human carbonic anhydrase IX (hCA IX) is a tumour-associated enzyme present in a limited number of normal tissues, but overexpressed in several malignant human tumours. It is a transmembrane protein, where the extracellular region consists of a greatly investigated catalytic CA domain and a much less investigated proteoglycan-like (PG) domain. Considering its important role in tumour biology, here, we report for the first time the full characterization of the PG domain, providing insights into its structural and functional features. In particular, this domain has been produced at high yields in bacterial cells and characterized by means of biochemical, biophysical and molecular dynamics studies. Results show that it belongs to the family of intrinsically disordered proteins, being globally unfolded with only some local residual polyproline II secondary structure. The observed conformational flexibility may have several important roles in tumour progression, facilitating interactions of hCA IX with partner proteins assisting tumour spreading and progression.     

Amyloid precursor protein and its homologues: a family of proteolysis-dependent receptors

The Alzheimer’s amyloid precursor protein (APP) belongs to a conserved gene family that also includes the mammalian APLP1 and APLP2, the Drosophila APPL, and the C. elegans APL-1. The biological function of APP is still not fully clear. However, it is known that the APP family proteins have redundant and partly overlapping functions, which demonstrates the importance of studying all APP family members to gain a more complete picture. When APP was first cloned, it was speculated that it could function as a receptor. This theory has been further substantiated by studies showing that APP and its homologues bind both extracellular ligands and intracellular adaptor proteins. The APP family proteins undergo regulated intramembrane proteolysis (RIP), generating secreted and cytoplasmic fragments that have been ascribed different functions. In this review, we will discuss the APP family with focus on biological functions, binding partners, and regulated processing.     

The ATP-binding cassette family: a structural perspective

The ATP-binding cassette family is one of the largest groupings of membrane proteins, moving allocrites across lipid membranes, using energy from ATP. In bacteria, they reside in the inner membrane and are involved in both uptake and export. In eukaryotes, these transporters reside in the cell’s internal membranes as well as in the plasma membrane and are unidirectional—out of the cytoplasm. The range of substances that these proteins can transport is huge, which makes them interesting for structure–function studies. Moreover, their abundance in nature has made them targets for structural proteomics consortia. There are eight independent structures for ATP-binding cassette transporters, making this one of the best characterised membrane protein families. Our understanding of the mechanism of transport across membranes and membrane protein structure in general has been enhanced by recent developments for this family.     

Uteroglobin: a novel cytokine?

Blastokinin or uteroglobin (UG) is a steroid-inducible, evolutionarily conserved, multifunctional protein secreted by the mucosal epithelial of virtually all mammals. It is present in the blood and in other body fluids including urine. An antigen immunoreactive to UG antibody is also detectable in the mucosal epithelia of all vertebrates. UG-binding proteins (putative receptor), expressed on several normal and cancer cell types, have been characterized. The human UG gene is mapped to chromosome 11q12.2 13.1, a region that is frequently rearranged or deleted in many cancers. The generation of UG knockout mice revealed that disruption of this gene causes: (i) severe renal disease due to an abnormal deposition of fibronectin and collagen in the glomeruli; (ii) predisposition to a high incidence of malignancies; and (iii) a lack of polychlorinated biphenyl binding and increased oxygen toxicity in the lungs. The mechanism(s) of UG action is likely to be even more complex as it also functions via a putative receptor-mediated pathway that has not yet been clearly defined. Molecular characterization of the UG receptor and signal transduction via this receptor pathway may show that this protein belongs to a novel cytokine/chemokine family.     

The function of apolipoproteins L

The function of the proteins of the apolipoprotein L (apoL) family is largely unknown. These proteins are classically thought to be involved in lipid transport and metabolism, mainly due to the initial discovery that a secreted member of the family, apoL-I, is associated with high-density lipoprotein particles. However, the other members of the family are believed to be intracellular. The recent unravelling of the mechanism by which apoL-I kills African trypanosomes, as well as the increasing evidence for modulation of apoL expression in various pathological processes, provides new insights about the functions of these proteins. ApoLs share structural and functional similarities with proteins of the Bcl-2 family. Based on the activity of apoL-I in trypanosomes and the comparison with Bcl-2 proteins, we propose that apoLs could function as ion channels of intracellular membranes and be involved in mechanisms triggering programmed cell death. Received 28 February 2006; received after revision 18 May 2006; accepted 2 June 2006