Proteins of the kidney microvillus membrane. Identification of subunits after sodium dodecylsullphate/polyacrylamide-gel electrophoresis

Abstract

The proteins of microvilli prepared from pig kidney were analyzed by polyacrylamide-gel electrophoresis in the presence of sodium dodecyl sulfate. The typical pattern stained for protein revealed 5 major bands, 4 of which also stained for carbohydrate, and about 15 minor bands. For descriptive purposes the bands were designated numerically by their apparent MW (.times. 10-3). Well-characterized proteins were identified with 4 of the 5 major bands. Dipeptidyl peptidase IV, a serine proteinase that may be specifically labeled with di-isopropyl [32P]phosphorofluoridate, was assigned to band 130. Aminopeptidase M was assigned to band 160, though when released from the membrane by a proteinase, this protein comprises 3 polypeptides each of lower apparent MW than the native enzyme. Neutral endopeptidase can be assigned to band 95 and actin to band 42. The 5th major band (180) is an extrinsic glycoprotein that has not been identified with any microvillus enzyme activity. These 4 proteins contribute 21% of the microvillus-membrane protein. Kidney microvillus actin was characterized by a variety of properties and was similar to muscle actin. A computer analysis of the gel pattern indicates that it comprises 9.0% of the microvillus protein. Myosin is not present in the microvillus, but another protein associated with band 95, with properties that distinguish it from neutral endopeptidase, was tentatively identified as .alpha.-actinin. Alkaline phosphatase was identified as a monomeric polypeptide with an apparent MW of 80,000; it is a minor protein of the microvillus and is not discernible as a discrete band in the gel pattern. A model of the organization of the microvillus protein is suggested.