Deamidation and isoaspartate formation in proteins: unwanted alterations or surreptitious signals?

Formation of -linked Asp-Xaa peptide bonds – isoaspartyl (isoAsp) sites – arise in proteins via succinimide–linked deamidation of asparagine or dehydration of aspartate, reactions which represent a major source of spontaneous protein damage under physiological conditions. Accumulation of atypical isoaspartyl sites is minimized in vivo by the activity of protein L-isoaspartyl O–methyltransferase (PIMT), which regenerates a normal peptide bond. Loss of PIMT has harmful consequences, especially in neurons; thus, formation of isoAsp sites and their subsequent correction by PIMT is widely believed to constitute an important pathway of protein damage and repair. Recent evidence is mounting, however, that deamidation and isoaspartate formation may, in some instances, constitute a novel mechanism for intentional modification of protein structure. Herein we describe the mechanism of Asx rearrangement, summarize the evidence that PIMT serves an important repair function, and then focus on emerging evidence that deamidation and isoAsp formation may sometimes have a useful function.Received 16 October 2002; received after revision 11 December 2002; accepted 12 December 2002     

Memory

Recent research in a variety of systems indicates that memory formation can involve the activation of a wide range of molecular cascades. In assessing this recent work it is clear that no single cascade is uniquely important for all forms of memory, nor is a single form of memory uniquely dependent on a single cascade. Rather, it appears that molecular networks are differentially engaged in the induction of various forms of memory. Despite this highly interactive array of possible cascades, specific 'molecular nodes' have emerged as critical regulatory points in memory formation. Functionally, these nodes can operate in two sequential steps, beginning with a convergence of inputs which coordinately influence the activation state of the node, in which the nature of stimulation determines the dynamics of nodal activity, followed by a divergence of substrate selection, in which the node serves as a gateway that activates specific downstream effectors. Finally, specific nodes can be differentially engaged (i.e. have different 'weights') depending upon the nature and pattern of the activating stimulus. The marine mollusk Aplysia has proven useful for a molecular analysis of memory formation. We will use this system to highlight some of the molecular strategies employed by the nervous system in the formation of memory for sensitization, and we will focus on extracellular signal-related kinase as a candidate node integral to these processes.