Kinetics of desmosome assembly in Madin-Darby canine kidney epithelial cells: temporal and spatial regulation of desmoplakin organization and stabilization upon cell-cell contact. II. Morphological analysis

Biochemical analysis of the kinetics of assembly of two cytoplasmic plaque proteins of the desmosome, desmoplakins I (250,000 Mr) and II (215,000 Mr), in Madin-Darby canine kidney (MDCK) epithelial cells, demonstrated that these proteins exist in a soluble and insoluble pool, as defined by their extract ability in a Triton X-100 high salt buffer (CSK buffer). Upon cell-cell contact, there is a rapid increase in the capacity of the insoluble pool at the expense of the soluble pool; subsequently, the insoluble pool is stabilized, while proteins remaining in the soluble pool continue to be degraded rapidly (Pasdar, M., and W. J. Nelson. 1988. J. Cell Biol. 106:677-685). In this paper, we have sought to determine the spatial distribution of the soluble and insoluble pools of desmoplakins I and II, and their organization in the absence and presence of cell-cell contact by using differential extraction procedures and indirect immunofluorescence microscopy. In the absence of cell-cell contact, two morphologically and spatially distinct patterns of staining of desmoplakins I and II were observed: a pattern of discrete spots in the cytoplasm and perinuclear region, which is insoluble in CSK buffer; and a pattern of diffuse perinuclear staining, which is soluble in CSK buffer, but which is preserved when cells are fixed in 100% methanol at -20 degrees C. Upon cell-cell contact, in the absence or presence of protein synthesis, the punctate staining pattern of desmoplakins I and II is cleared rapidly and efficiently from the cytoplasm to the plasma membrane in areas of cell- cell contact (less than 180 min). The distribution of the diffuse perinuclear staining pattern remains relatively unchanged and becomes the principal form of desmoplakins I and II in the cytoplasm 180 min after induction of cell-cell contact. Thereafter, the relative intensity of staining of the diffuse pattern gradually diminishes and is completely absent 2-3 d after induction of cell-cell contact. Significantly, double immunofluorescence shows that during desmosome assembly on the plasma membrane both staining patterns coincide with a subpopulation of cytokeratin intermediate filaments. Taken together with the preceding biochemical analysis, we suggest that the assembly of desmoplakins I and II in MDCK epithelial cells is regulated at three discrete stages during the formation of desmosomes.