Presence of free hydrophobic peptides in the brush border and basolateral membranes of pig enterocytes.

Short peptides containing approx. 60% of hydrophobic amino acids have been extracted by chloroform/methanol from purified brush border and basolateral membranes of pig enterocytes. These peptides can be separated from membrane lipids by thin-layer chromatography on Kieselgel plates using chloroform/methanol/water as developer. Their molecular weight is approx. 8000 as judged by SDS-gel electrophoresis. But, this value may be overestimated. They are devoid of cystine and methionine. They contain no N-terminal amino acid detectable by the dansyl and Edman degradation techniques. Extraction of papain-treated, right side out brush border vesicles led to mixtures containing the above peptides and the anchors which normally bind a variety of hydrolases to the external surface of the brush border. Peptides and anchors could not be separated by high performance thin-layer chromatography and SDS-gel electrophoresis. Their amino acid compositions were similar. However, several lines of evidence did not support the assumption that the peptides existing in non-treated brush border membranes can be identified to anchors left inside the bilayer after proteolytic cleavage of surface hydrolases. It is not yet known whether these peptides represent other hydrophobic fragments (leader or stop-transfer sequence, for instance) left in the membrane during the co-translational processing of certain proteins or constitute an independent population of molecules.