Protein Assembly in the ER

If our knowledge of the molecular mechanisms responsible for the recognition and membrane integration of transmembrane segments is thus quite limited, the situation is significantly better when we ask about the substrate, i. e., the characteristics of the nascent chain that have an impact on the assembly process. Starting with the hydrophobic transmembrane segment itself, many studies have shown that there is a minimum 

We have studied the sequence determinants for helical hairpin formation during the insertion of a model membrane protein into the ER membrane. To simplify the problem, we engineered a 40-residue long poly(Leu) stretch into a membrane protein that inserts readily into ER-derived microsomes when expressed in vitro (Fig. 2A). Asn-X-Thr acceptor sites for N-linked glycosylation were used as topological markers, as they can only be modified when located in the 

Interestingly, it appears that it is easier to induce the formation of a helical hairpin with the tight turn on the lumenal side of the ER membrane than one with the opposite orientation (cytoplasmic turn) i.e., whereas a single Pro is enough to convert the 40-residues long poly (Leu) stretch to a helical hairpin with a lumenal turn, three consecutive prolines are needed for a helical hairpin with a cytoplasmic turn to form (Saaf et al., 2000) (Fig. 2B). If one only considers simple protein—lipid interactions there is no obvious thermodynamic reason why this should be so instead, we favor the view that this reflects a constraint on helical hairpin structure imposed by the Sec machinery. 

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