Poly stretch, membrane protein

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... 

The important bacterial storage material poly-hydroxybutyric acid is related metabolically and structurally to the lipids. This highly reduced polymer is made up of D-(3-hydroxybutyric acid units in ester linkage, about 1500 residues being present per chain. The structure is that of a compact right-handed coil with a twofold screw axis and a pitch of 0.60 nm.a Within bacteria it often occurs in thin lamellae 5.0 nm thick. Since a chain of 1500 residues stretches to 440 nm, there must be 88 folds in a single chain. Present in both cytoplasmic granules and in membranes,b polyhydroxybutyrate can account for as much as 50% of the total carbon of some bacterial In E. coli and many other bacteria polyhydroxybutyrate is present in a lower molecular mass form bound to calcium polyphosphates, proteins, or other macromolecules.d e It has also been extracted from bovine serum albumin and may be ubiquitous in both eukaryotes and prokaryotes.d/e The polymer may function in formation of Ca2+ channels in membranes.b/d... 

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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