THE CYTOSKELETON IN DEVELOPMENT OF EPITHELIAL-CELL POLARITY

The polarization of intestinal epithelial cells and the stereotypic ar rangement of their actin-based cytoskeleton have made these epithelia an excellent system to explore the organization and formation of a cor tical actin-based cytoskeleton. Through a combined morphological and b iochemical analysis, the molecular arrangement of many of the componen ts of the brush border has been elucidated. Study of brush border asse mbly in the Crypts of Lieberkuhn suggests that cytoskeletal mRNA and p rotein expression, as well as morphological development, occur rapidly following cell differentiation. Protein kinases appear to be importan t regulators of intestinal cell growth, for differentiating cells in t he crypts possess 15-fold higher levels of tyrosine phosphorylated pro teins than differentiated cells of the villus. One of these kinases, p p60(c-src), has a 4- to 7-fold higher activity in crypts and increased association with the cytoskeleton than it has in villus cells. The de velopment and maintenance of polarization in epithelial cells require the targeting and transport of specific proteins to the apical and bas olateral plasma membrane. It has been proposed that a dynein-like, mic rotubule-based motor is involved in the transport of apically directed materials from the trans-Golgi to the apical plasma membrane. However , microtubules do not reach the plasma membrane, but terminate below t he actin-rich network of filaments comprising the terminal web. We pro pose that vesicles translocate from the Golgi to the apical cytoplasm along microtubules using dynein, and then move through the terminal we b to reach the apical plasma membrane using the actin-based motor myos in-I. Our isolation of Golgi-derived vesicles possessing both myosin-I and dynein on their cytoplasmic surface is consistent with this hypot hesis.