Membrane protein acylation

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Abstract
Mycoplasmas are small bacteria without a cell wall, often found as surface parasites on eukaryotic cells. Of the more than 200 membrane proteins from Acholeplasma laidlawii resolved by two‐dimensional PAGE, 23 were covalently modified with acyl chains. These acyl proteins had lower pI values than average and were all labelled by different exogenously supplied radioactive fatty acids attached by O‐ester bonds. The fatty acids were selectively incorporated in the order myristic acid (14:0) > palmitic acid (16:0) > stearic acid (18:0) > oleic acid (18:1). However, endogenously synthesised saturated fatty acids, most of which were 16:0, were preferred over the supplied ones. A fraction of the exogenous 14:0 was elongated to 16:0. Absence of saturated fatty acids increased the incorporation of 18:1. The maximum extent of modification was one acyl chain for protein T2, on the exterior surface and two acyl chains for protein D12, spanning the membrane. Exogenously supplied fatty acids were incorporated into membrane lipids in proportion to their occurrence. However, the acylated proteins always contained 8–10 times more saturated chains than did the lipids. When exogenously supplied, all A. laidlawii polar membrane lipids could donate acyl chains to the acylated proteins but the neutral fraction (fatty acids and diacylglycerol) was most efficient. An incorporation into the acylated proteins of labelled cysteine, but not glucose or glycerol, was observed. Acylated proteins with different chains interacted similarily with a Triton X‐114 detergent phase, and no full‐size proteins (or acylated fragments) were released from cells by proteolytic enzymes. The results indicate an anchoring with peptide segments in addition to the acyl chains. Both 14:0 and 16:0 were attached at one end of both T2 and D12, but the N‐terminal methionine of T2 was not acylated. The extent of modification and preference for saturated chains in the A. laidlawii membrane acylated proteins is more similar to eukaryotic than to eubacterial proteins.

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