Organization of actin filaments and immunocolocalization of alpha-actinin in the connecting cilium of rat photoreceptors.

A small discrete concentration of actin filaments in the connecting cilium of vertebrate photoreceptors appears to have a role in the morphogenesis of the phototransductive disk membranes (Williams et al., '88). We have visualized these actin filaments in rat rod photoreceptors by decorating them with myosin subfragment-1. At the site of disk morphogenesis, we observed a cluster of short filaments, with various orientations and their faster growing (barbed) ends at the ciliary plasma membrane. Their association with the liplike structure of an early nascent disk is consistent with their apparent involvement in the initiation of disk morphogenesis. A few longer decorated filaments extended along the core the connecting cilium, away from the site of disk morphogenesis, implying that they might have some function other than the shaping of a new disk. Most of the antiactin label was found in the region of the short filaments. The alpha-actinin immunolabel coincided with that of actin, suggesting that the filaments may be crosslinked by alpha-actinin.