Cloning and characterization of mouse brush border myosin-I in adult and embryonic intestine.

Brush border myosin-I is a class I myosin with calmodulin light chains that has been identified in several vertebrate species. In chicken, it is exclusively expressed in intestinal epithelial cells where it forms spirally arrayed bridges that tether the microvillar actin bundle to the membrane. To facilitate future knockout strategies, we have isolated mouse brush border myosin-I cDNA and genomic clones. The deduced primary structure of mouse brush border myosin-I is homologous to other known brush border myosins-I. Northern blot, immunoblot, and immunolocalization studies indicate that the intestine-specific and subcellular localization profile of mouse brush border myosin-I are comparable to that determined for other brush border myosins-I. Northern analysis during embryogenesis revealed a 3.9-kb transcript first detected in 15-day embryos. This is in marked contrast to chicken, where brush border myosin-I expression begins early in embryogenesis. In situ localization in 17-day embryos indicated that RNA expression is restricted to the intestine. Protein expression is first detected in 16-day embryos with decreasing levels observed in a proximal to distal fashion. Immunolocalization in embryonic intestine revealed that brush border myosin-I is evenly distributed on both apical and basolateral membrane domains. There is also pronounced localization to a supranuclear region, presumably the Golgi apparatus. This suggests that brush border myosin-I may be targeted to the plasma membrane on Golgi-derived vesicles rather than by direct targeting to microvillar actin cores.