Brush border myosin-I microinjected into cultured cells is targeted to actin-containing surface structures.
The isolated intestinal microvillus cytoskeleton (core) consists of four major proteins: actin, villin, fimbrin and brush border myosin-I. These proteins can assemble in vitro into structures resembling native microvillus cores. Of these components, villin and brush border myosin-I show tissue-specific expression, so they may be involved in the morphogenesis of intestinal microvilli. When introduced into cultured cells that normally lack the protein, villin induces a reorganization of the actin filaments to generate large surface microvilli. Here we examine the consequences of microinjecting brush border myosin-I either alone or together with villin into cultured fibroblasts. Injection of brush border myosin-I has no discernible effect on the overall morphology of the cells, but does become localized to either normal or villin-induced microvilli and other surface structures containing an actin cytoskeleton. Since some endogenous myosin-Is have been found associated with cytoplasmic vesicles, these results show that brush border myosin-I has a domain that specifically targets it to the plasma membrane in both intestinal and cultured cell systems. Ultrastructural examination of microvilli on control cultured cells revealed that they contain a far more highly ordered bundle of microfilaments than had been previously appreciated. The actin filaments in microvilli of villin-injected cells appeared to be more tightly cross-linked when examined by thin-section electron microscopy. In intestinal microvilli, the core bundle is separated from the plasma membrane by about 30 nm due to the presence of brush border myosin-I.(ABSTRACT TRUNCATED AT 250 WORDS)
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Section of Biochemistry, Molecular and Cell Biology, Cornell University, Ithaca, NY 14853.
Bretscher
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D016475:3T3 Cells / Q000378:metabolism* / Q000648:ultrastructure; D000199:Actins / Q000378:metabolism*; D000818:Animals; D002352:Carrier Proteins / Q000008:administration & dosage / Q000378:metabolism; D002478:Cells, Cultured; D003599:Cytoskeleton / Q000378:metabolism* / Q000648:ultrastructure; D005455:Fluorescent Antibody Technique; D007413:Intestinal Mucosa / Q000648:ultrastructure*; D051379:Mice; D008840:Microfilament Proteins / Q000008:administration & dosage / Q000378:metabolism; D008845:Microinjections; D008871:Microvilli / Q000737:chemistry; D009218:Myosins / Q000008:administration & dosage / Q000378:metabolism*
D000199:Actins; D002352:Carrier Proteins; D008840:Microfilament Proteins; C026615:villin; D009218:Myosins