Distribution of actin and the actin-associated proteins myosin, tropomyosin, alpha-actinin, vinculin, and villin in rat and bovine exocrine glands.

Actin, myosin, and the actin-associated proteins tropomyosin, alpha-actinin, vinculin, and villin were localized in acinar cells of rat and bovine pancreas, parotid, and prostate glands by means of immunofluorescent staining of both frozen tissue sections and semithin sections of quick-frozen, freeze-dried, and plastic-embedded tissues. Antibodies to actin, myosin, tropomyosin, alpha-actinin, and villin reacted strongly with a narrow cytoplasmic band extending beneath the luminal border of acinar cells. The presence of villin, which has so far been demonstrated only in intestinal and kidney brush border, was further confirmed by antibody staining of blotted electrophoresis gels of whole acinar cell extracts. Fluorescently labelled phalloidin, which reacts specifically with F-actin, gave similar staining, within the cell apex to that obtained with antibodies to actin, myosin, tropomyosin, alpha-actinin, and villin. In contrast, immunostaining with antibodies to vinculin was restricted to the area of the junctional complex. Ultrastructurally, the apical immunoreactive band corresponded to a dense web composed of interwoven microfilaments, which could be decorated with heavy meromyosin. Outside this apical terminal web, antibodies to myosin and tropomyosin gave only a weak immunostaining (confined to the lateral cell borders) whereas antibodies to actin and alpha-actinin led to a rather strong bead-like staining along the lateral and basal cell membrane most probably marking microfilament-associated desmosomes. Anti-villin immunofluorescence was confined to the apical terminal web. It is suggested that the apical terminal web is important for the control of transport and access of secretory granules to the luminal plasma membrane and that villin, which is known to bundle or sever actin filaments in a Ca(++)-dependent manner, might participate in the regulation of actin polymerization within this strategically located network of contractile proteins.