Unity in function in the absence of consensus in sequence: role of leader peptides in export

Passage of proteins across membranes during export from their site of synthesis to their final destination is mediated by leader peptides that paradoxically exhibit a unity of function in spite of a diversity of sequence. These leader peptides act in at least two stages of the export process: at entry into the pathway and subsequently during translocation across the membrane. How selectivity is imposed on the system in the absence of a consensus among the sequences of leader peptides is the main issue discussed here.     

Modulation of folding pathways of exported proteins by the leader sequence

Leader peptides that function to direct export of proteins through membranes have some common features but exhibit a remarkable sequence diversity. Thus there is some question whether leader peptides exert their function through conventional stereospecific protein-protein interaction. Here it is shown that the leader peptides retarded the folding of precursor maltose-binding protein and ribose-binding protein from Escherichia coli. This kinetic effect may be crucial in allowing precursors to enter the export pathway.     

No specific recognition of leader peptide by SecB, a chaperone involved in protein export

Most proteins destined for export from Escherichia coli are made as precursors containing amino-terminal leader sequences that are essential for export and that are removed during the process. The initial step in export of a subset of proteins, which includes maltose-binding protein, is binding of the precursor by the molecular chaperone SecB. This work shows directly that SecB binds with high affinity to unfolded maltose-binding protein but does not specifically recognize and bind the leader. Rather, the leader modulates folding to expose elements in the remainder of the polypeptide that are recognized by SecB.     

领导群体知识结构的优化配置

科学的领导群体的知识结构应当是“软”知识与“硬”知识的结合,以“软”知识为主,专业知识与一般知识结合,以专业知识为主;书本知识与经验知识结合,以经验知识为主。知识结构应不断更新。