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<jats:p> The plant growth regulator abscisic acid (ABA) is formed by the oxidative cleavage of an epoxy-carotenoid. The synthesis of other apocarotenoids, such as vitamin A in animals, may occur by a similar mechanism. In ABA biosynthesis, oxidative cleavage is the first committed reaction and is believed to be the key regulatory step. A new ABA-deficient mutant of maize has been identified and the corresponding gene, <jats:italic>Vp14</jats:italic> , has been cloned. The recombinant VP14 protein catalyzes the cleavage of 9- <jats:italic>cis</jats:italic> -epoxy-carotenoids to form C <jats:sub>25</jats:sub> apo-aldehydes and xanthoxin, a precursor of ABA in higher plants. </jats:p>

<jats:p>The human cytomegalovirus encodes a β-chemokine receptor (US28) that is distantly related to the human chemokine receptors CCR5 and CXCR4, which also serve as cofactors for the entry into cells of human immunodeficiency virus–type 1 (HIV-1). Like CCR5, US28 allowed infection of CD4-positive human cell lines by primary isolates of HIV-1 and HIV-2, as well as fusion of these cell lines with cells expressing the viral envelope proteins. In addition, US28 mediated infection by cell line–adapted HIV-1 for which CXCR4 was an entry cofactor.</jats:p>

<jats:p> Various receptors coupled to the heterotrimeric guanine nucleotide-binding protein Gq/11 stimulate formation of inositol-1,4,5-trisphosphate (IP <jats:sub>3</jats:sub> ). Activation of these receptors also induces protein tyrosine phosphorylation. Formation of IP <jats:sub>3</jats:sub> in response to stimulated receptors that couple to Gq/11 was blocked by protein tyrosine kinase inhibitors. These inhibitors appeared to act before activation of Gq/11. Moreover, stimulation of receptors coupled to Gq/11 induced phosphorylation on a tyrosine residue (Tyr <jats:sup>356</jats:sup> ) of the Gα <jats:sub>q/11</jats:sub> subunit, and this tyrosine phosphorylation event was essential for Gq/11 activation. Tyrosine phosphorylation of Gα <jats:sub>q/11</jats:sub> induced changes in its interaction with receptors. Therefore, tyrosine phosphorylation of Gα <jats:sub>q/11</jats:sub> appears to regulate the activation of Gq/11 protein. </jats:p>