Glycosylation Mapping Of The Interaction Between Topogenic Sequences And The Er Translocase

Abstract

Many integral membrane proteins span the hydrophobic core of a membrane with one or more a-helical segments each consisting of about 20 hydrophobic amino acids. The orientation of the hydrophobic stretch in the membrane is determined by its flanking amino acids. In general there are more positively charged residues present in cytoplasmic loops than in extra-cytoplasmic ones (von Heijne, 1986). In both prokaryotic and eukaryotic cells, proteins are targeted for secretion by N-terminal signal sequences with a common basic design: a positively charged N-terminus, a central hydrophobic stretch, and a C-terminal cleavage region that serves as a recognition site for the signal peptidase enzyme (von Heijne, 1985). Signal sequences from prokaryotes and eukaryotes look very similar and are often functionally interchangeable. They are essential for the efficient and selective targeting of the nascent protein chains either to the endoplasmic reticulum (in eukaryotes) or to the cytoplasmic membrane (in prokaryotes) (Gierasch, 1989). Signal sequences also play a central role in the interaction with the translocation machinery of the cell and in the translocation of the polypeptide chains across the membrane. Proteins are co-translationally translocated across the endoplasmic reticulum (ER) membrane. In eukaryotes two kinds of topogenic sequences are important for assembly in ER membrane: Those that initiate translocation such as signal peptides (SPs) and signalanchor sequences (SAs) and those that halt translocation, stop-transfer signals (STs). Signal peptides and signal anchor sequences differ in that SA sequences tend to have longer hydrophobic cores and lack a signal peptidase cleavage site (von Heijne, 1988).