Stimulation of B lymphocytes through their antigen receptor (BCR) results in rapid increases in tyrosine phosphorylation on a number of proteins and induces both an increase of phosphatidylinositol and mobilization of cytoplasmic free calcium. The BCR associates with two classes of tyrosine kinase: Src‐family kinase (Lyn, Fyn, Blk or Lck) and Syk kinase. To dissect the functional roles of these two types of kinase in BCR signaling, lyn‐negative and syk‐negative B cell lines were established. Syk‐deficient B cells abolished the tyrosine phosphorylation of phospholipase C‐gamma 2, resulting in the loss of both inositol 1,4,5‐trisphosphate (IP3) generation and calcium mobilization upon receptor stimulation. Crosslinking of BCR on Lyn‐deficient cells evoked a delayed and slow Ca2+ mobilization, despite the normal kinetics of IP3 turnover. These results demonstrate that Syk mediates IP3 generation, whereas Lyn regulates Ca2+ mobilization through a process independent of IP3 generation.

A group of membrane-associated guanine nucleotide binding proteins (G-proteins) are essential for transducing signals generated at cell-surface receptors into changes in cellular function and metabolism. These proteins are a complex of three subunits designated alpha, beta and gamma. The alpha-subunit is responsible for binding guanine nucleotides and seems to be characteristic of each protein. Transducin, a member of this protein family, mediates visual transduction by coupling the signal of photolysed rhodopsin with activation of a cyclic GMP phosphodiesterase. We have now cloned and sequenced the complementary DNA encoding the alpha-subunit of bovine retinal transducin and from this we have deduced the complete amino-acid sequence. The transducin alpha-subunit shares several homologous amino-acid sequences with ras gene products. The homologous segments correspond mostly to the regions thought to be involved in the guanine nucleotide binding and GTPase activity of ras proteins and to the ADP-ribosylation sites of the transducin alpha-subunit.

Interactions of G-protein ␣ (G␣) and ␤␥ subunits (G␤␥) with N-(␣ 1B ) and P/Q-type (␣ 1A ) Ca 2؉ channels were investigated using the Xenopus oocyte expression system. G i3 ␣ was found to inhibit both N-and P/Q-type channels by receptor agonists, whereas G␤ 1 ␥ 2 was responsible for prepulse facilitation of N-type channels. L-type channels (␣ 1C ) were not regulated by G␣ or G␤␥. For N-type, prepulse facilitation mediated via G␤␥ was impaired when the cytoplasmic I-II loop (loop 1) was deleted or replaced with the ␣ 1C loop 1. G␣-mediated inhibitions were also impaired by substitution of the ␣ 1C loop 1, but only when the C terminus was deleted. For P/Q-type, by contrast, deletion of the C terminus alone diminished G␣-mediated inhibition. Moreover, a chimera of L-type with the ␣ 1B loop 1 gained G␤␥-dependent facilitation, whereas an L-type chimera with the Nor P/Q-type C terminus gained G␣-mediated inhibition. These findings provide evidence that loop 1 of N-type channels is a regulatory site for G␤␥ and the C termini of P/Q-and N-types for G␣.

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