Centrin in the photoreceptor cells of mammalian retinae

Photoreceptor cells of vertebrate retinae are highly specialized ciliary cells. Their non-motile ciliated structure is restricted to the so-called connecting cilium at the joint between the light sensitive outer segment and the metabolically active inner segment. Extensive bidirectional intracellular transport between both segments is forced to occur through this tight connecting cilium. In the present study it is shown that the CA2+-binding, phospho-protein centrin is present in mammalian retinae. Western blot and immunoprecipitation reveal that anti-centrin antibodies react with purified photoreceptor cell fractions of retinae in bands at a molecular weight of 20 kDa, the molecular weight of centrins found in other cells. Indirect immunofluorescence analysis of cryosections through retinae of different mammalian species show that centrin is present only in centrosomes and basal bodies but also more extensively at the linkage between the inner and the outer segment of the photoreceptor cells. Immunocytological studies on isolated rod cells and immunoelectron microscopy clearly demonstrate a unique presence of centrin in the connecting cilium of photoreceptor cells. High molecular identity between centrins in lower eukaryotes and mammals indicates that centrin may play a role in cellular motility and/or in microtubule severing in the mammalian retina.