Protein targeting and degradation are coupled for elimination of mislocalized proteins

A substantial proportion of the genome encodes membrane proteins that are delivered to the endoplasmic reticulum by dedicated targeting pathways. Membrane proteins that fail targeting must be rapidly degraded to avoid aggregation and disruption of cytosolic protein homeostasis. The mechanisms of mislocalized protein (MLP) degradation are unknown. Here we reconstitute MLP degradation in vitro to identify factors involved in this pathway. We find that nascent membrane proteins tethered to ribosomes are not substrates for ubiquitination unless they are released into the cytosol. Their inappropriate release results in capture by the Bag6 complex, a recently identified ribosome-associating chaperone. Bag6-complex-mediated capture depends on the presence of unprocessed or non-inserted hydrophobic domains that distinguish MLPs from potential cytosolic proteins. A subset of these Bag6 complex 'clients' are transferred to TRC40 for insertion into the membrane, whereas the remainder are rapidly ubiquitinated. Depletion of the Bag6 complex selectively impairs the efficient ubiquitination of MLPs. Thus, by its presence on ribosomes that are synthesizing nascent membrane proteins, the Bag6 complex links targeting and ubiquitination pathways. We propose that such coupling allows the fast tracking of MLPs for degradation without futile engagement of the cytosolic folding machinery.     

Distant metastasis occurs late during the genetic evolution of pancreatic cancer

Metastasis, the dissemination and growth of neoplastic cells in an organ distinct from that in which they originated, is the most common cause of death in cancer patients. This is particularly true for pancreatic cancers, where most patients are diagnosed with metastatic disease and few show a sustained response to chemotherapy or radiation therapy. Whether the dismal prognosis of patients with pancreatic cancer compared to patients with other types of cancer is a result of late diagnosis or early dissemination of disease to distant organs is not known. Here we rely on data generated by sequencing the genomes of seven pancreatic cancer metastases to evaluate the clonal relationships among primary and metastatic cancers. We find that clonal populations that give rise to distant metastases are represented within the primary carcinoma, but these clones are genetically evolved from the original parental, non-metastatic clone. Thus, genetic heterogeneity of metastases reflects that within the primary carcinoma. A quantitative analysis of the timing of the genetic evolution of pancreatic cancer was performed, indicating at least a decade between the occurrence of the initiating mutation and the birth of the parental, non-metastatic founder cell. At least five more years are required for the acquisition of metastatic ability and patients die an average of two years thereafter. These data provide novel insights into the genetic features underlying pancreatic cancer progression and define a broad time window of opportunity for early detection to prevent deaths from metastatic disease.     

Human MutY: gene structure,protein functions and interactions,and role in carcinogenesis

Faithful maintenance of the genome is crucial to the individual and the species. Oxidative DNA damage, such as 8-oxo-7,8-dihydroguanine (8-oxoG), poses a major threat to genomic integrity. 8-OxoG can mispair with 2-deoxycytidine 5-triphosphate or with 2-deoxyadenosine triphosphate during DNA replication, forming C8-oxoG and A8-oxoG mispairs. Human MutY is responsible for recognition and removal of the inappropriately inserted adenine in an A8-oxoG mispair. If unrepaired, the A8-oxoG mispairs can result in deleterious C:G to A:T transversions. Human MutY functions in a postreplication repair pathway and is targeted to the newly synthesized daughter strand of DNA for removal of the adenine base. The human MutY protein is targeted to both the mitochondria and the nucleus and associates with the proliferating cell nuclear antigen, apurinic/ apyrimidinic endonuclease 1, replication protein A and mutS homolog 6 proteins. Mutations in the human MutY gene and defective activity of the human MutY protein have been detected in cancer. A direct correlation between defective A8-oxoG repair and increased levels of genomic 8-oxoG has now been established.Received 10 February 2003; received after revision 7 April 2003; accepted 14 April 2003