Selective incorporation of architectural proteins into terminally differentiated molluscan gill cilia

Incubation of excised gills from the bay scallop Aequipecten irradians with 3H-leucine demonstrates that many ciliary structural proteins can attain a degree of labeling approaching that previously reported for sea urchin or surf clam embryos undergoing ciliary turnover or regeneration. This labeling is not a consequence of any predominant incorporation into new cilia at the meristematic growth tips of the gill since tissue regions of varying maturity incorporate label into the same proteins at similar levels, with the most mature region having the highest incorporation. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorographic analysis of isolated cilia, separated into detergent-soluble membrane/matrix and detergent-insoluble 9+2 axoneme fractions, reveals that 1) tubulin in the membrane/matrix fraction is labeled whereas tubulin in the axoneme is not; 2) no labeled dynein heavy chains are seen in either fraction; 3) the most heavily labeled axonemal components do not appear to any significant extent in the membrane/matrix fraction; and 4) after thermal depolymerization of the microtubules, nearly all labeled proteins reside in the in-soluble ninefold ciliary remnant, the most prominent being tektin A, an integral component of outer doublet microtubules. Further fractionation of the remnant with sarkosyl-urea to produce tektin filaments demonstrates two solubility classes of tekin A, only the more soluble of which is labeled. Very similar selective architectural protein labeling patterns have been reported for steady-state cilia of sea urchin embryos, and this may indicate a widespread turnover or exchange mechanism characteristic of cilia heretofore considered static.