Converting a Marginally Hydrophobic Soluble Protein into a Membrane Protein

• delta-Helices are marginally hydrophobic a-helical segments in soluble proteins that exhibit certain sequence characteristics of transmembrane (TM) helices [Cunningham, F., Rath, A., Johnson, R. M. & Deber, C. M. (2009). Distinctions between hydrophobic helices in globular proteins and TM segments as factors in protein sorting. J. Biol. Chem., 284, 5395-402]. In order to better understand the difference between delta-helices and TM helices, we have studied the insertion of five TM-like delta-helices into dog pancreas microsomal membranes. Using model constructs in which an isolated delta-helix is engineered into a bona fide membrane protein, we find that, for two delta-helices originating from secreted proteins, at least three single-nucleotide mutations are necessary to obtain efficient membrane insertion, whereas one mutation is sufficient in a delta-helix from the cytosolic protein P450BM-3. We further find that only when the entire upstream region of the mutated delta-helix in the intact cytochrome P450BM-3 is deleted does a small fraction of the truncated protein insert into microsomes. Our results suggest that upstream portions of the polypeptide, as well as embedded charged residues, protect delta-helices in globular proteins from being recognized by the signal recognition particle-Sec61 endoplasmic-reticulum-targeting machinery and that delta-helices in secreted proteins are mutationally more distant from TM helices than delta-helices in cytosolic proteins.

Ämnesord

NATURVETENSKAP  -- Biologi (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences (hsv//eng)

Nyckelord

membrane proteins

membrane insertion

transmembrane helix

protein folding

protein localization

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