DISTINCT LOCALIZATIONS OF TROPOMYOSIN ISOFORMS IN LLC-PK1 EPITHELIAL-CELLS SUGGESTS SPECIALIZED FUNCTION AT CELL-CELL ADHESIONS

At least eight nonmuscle, nonbrain tropomyosin isoforms have been desc ribed. We used antibodies, microinjection, and transfection to charact erize their expression and localization in LLC-PK1 kidney epithelial c ells and compared them with other cells. Similar to primary enterocyte s, LLC-PK1 cells exhibited predominantly TM-1 and TM-3 of the high-mol ecular-weight (HMW) isoforms; TM-5 and TM-Sb of the low-molecular-weig ht (LMW) isoforms. Neither TM-4 nor TM-Sa was detectable in the LLC-PK 1 cells. Immunofluorescence studies revealed that HMW isoforms were lo calized only on stress fibers, not adhesion belts, whereas the adhesio n belts were stained by LMW isoform antibodies. When exogenous protein s are introduced either by transfection or microinjection, the HMW iso forms do not incorporate into the adhesion belt, whereas the LMW isofo rms can incorporate into the stress fibers, thus indicating there are different mechanisms at work for the selective localization. Temporal changes in the microfilament system of the LLC-PK1 cells were studied during differentiation in culture as defined by spectrin expression an d F-actin architecture. Western blot analysis indicated that TM-Sb is only expressed in the LLC-PK1 cells after a certain degree of maturati on in culture, which suggests isoform switching after the cell-cell co ntacts are developed. Collectively these results demonstrate that epit helial cells express a complex pattern of TM isoforms, which exhibit d ifferential localizations within the cells and different patterns of e xpression depending on their origin and stage of differentiation. The implication of differential localization of TM isoforms on their speci fic functions is discussed. Cell Motil. Cytoskeleton 40:393-407, 1998. (C) 1998 Wiley-Liss, Inc.