Calcium-dependent phosphatidylinositol phosphorylation in lamellibranch gill lateral cilia

Pure lateral (L) cilia may be separated from the remaining (R) cilia types of Mytilus edulis gill by serotonin activation after hypertonic shock. The two classes of cilia were permeabilized with 0.012% Triton X-100 and incubated with 32P-labeled ATP at low Ca++ (10(-7) M), where L cilia beat, or in high Ca++ (2-20 microM), where L cilia arrest but R cilia are active. The labeled cilia were separated into axoneme and membrane-matrix fractions by detergent extraction, subjected to SDS-PAGE on 5-15% gels, and autoradiographed. Neither cilia type undergoes Ca++-dependent phosphorylation of specific proteins, suggesting that neither Ca++-induced arrest in L cilia nor the Ca++ activation of other cilia is phosphorylation-dependent. However, lipid phosphorylation in L cilia is highly Ca++-dependent. Identified by thin-layer chromatography, the phospholipid that is phosphorylated in a Ca++-dependent manner is phosphatidylinositol 4-phosphate (PIP), yielding the 4,5-bisphosphate (PIP2). PIP2 increases at least 3-fold under Ca++-arrest conditions. Aequipecten gill lateral cilia, which require higher Ca++ levels for arrest, show even more striking changes. In both cases, the effect is maximal at micromolar Ca++ levels. Phosphorylation of other lipids is Ca++-independent. In the Ca++-insensitive or activated R cilia, PIP2 levels are intermediate, increasing only marginally with increased [Ca++]. The formation of PIP2 in response to Ca++, as opposed to its breakdown to form inositol 1,4,5-trisphosphate and diacylglycerol, may be characteristic of a Ca++ transport system. Mechanically sensitive, the L cilia arrest as a consequence of an inward flux of Ca++ ions, acting directly on the axoneme.(ABSTRACT TRUNCATED AT 250 WORDS)