Quantal tektin synthesis and ciliary length in sea-urchin embryos

Previous work using pulse-chase labelling of embryos from the sea-urchin Strongylocentrotus droebachiensis during ciliogenesis, regeneration or steady-state maintenance and elongation showed that a ciliary outer doublet microtubule-associated protein, originally termed component-20, was synthesized in a fixed or quantal amount. This suggested that the limited synthesis of component-20 might limit ciliary length, since the embryo has a large pool of most other ciliary components. Labelling experiments with S. purpuratus embryos now confirm quantal synthesis of component-20, while antibodies to S. purpuratus sperm flagellar tektins identify component-20 as the ciliary equivalent of the flagellar 55 x 10(3) Mr tektin, tektin A. Sequential pulse-chase labelling at various times prior to isolation of cilia proves that the high specific activity of this protein truly reflects de novo synthesis of a structurally stable protein and not rapid protein turnover. Embryos may be animalized by growth in the presence of zinc ions, resulting in cilia averaging nearly twice the normal 20 microns length. When these embryos are pulse-chase labelled during ciliary growth and elongation, labelling of tektin A is proportional to the greater ciliary length, as is the pool of labelled but unincorporated tektins and other minor proteins. Deciliated animalized and control embryos, pulse-chase labelled during their identical phases of ciliary regeneration, incorporate labelled tektin A to the same extent and have similar pools of unincorporated proteins. The correlation of enhanced tektin A synthesis with increased ciliary length and the coincidence of tektin A synthesis with ciliary elongation are observations consistent with the hypothesis that tektin A is a ciliary length-limiting structural element.