On the basis of the structural organization of actin and myosin in the brush border and the nature of microvillar contraction in-vitro, we had proposed a simple sarcomere model for the generation of microvillar movements. However, recent results from our laboratory have led us to have serious doubts regarding any model which proposes that the microvillus core is a rigid rod that is moved about in some way by interactions with myosin at its basal end in the terminal web. We have observed that the same levels of Ca++ that activate contractility also induce a drastic, but reversible, disruption of microvillus core filaments, which is mediated by the 'cutting action' of microvillus 95K on those filaments. We are now left with a puzzle regarding the relationship between Ca++-dependent solation of core filaments in the microvillus and Ca++-dependent activation of contractility in the terminal web. One clue that we have pursued in hopes of deciphering the relationship between these two potentially antagonistic effects of Ca++ on brush-border cytoskeletal structure is the observation that the rootlet end of the microvillus core does not solate in the presence of Ca++. As noted above, only the rootlet end of the microvillus core contains tropomyosin, thus raising the possibility that this protein 'protects' core filaments from solation at the site of the interaction with myosin. In this paper we present the results of studies prompted by these observations, which demonstrate that tropomyosin protects actin filaments from Ca++-dependent fragmentation by microvillus 95K. We also describe experiments that provide considerable insight into the way 95K 'cuts' actin filaments in the presence of Ca++, using a novel assay for analysis of the effects of actin-binding proteins on filament structure and polymerization.
All Science Journal Classification (ASJC) codes
- Molecular Biology