Novel Gq alpha isoform is a candidate transducer of rhodopsin signaling in a Drosophila testes-autonomous pacemaker.

DGq is the alpha subunit of the heterotrimeric GTPase (G alpha), which couples rhodopsin to phospholipase C in Drosophila vision. We have uncovered three duplicated exons in dgq by scanning the GenBank data base for unrecognized coding sequences. These alternative exons encode sites involved in GTPase activity and G beta-binding, NorpA (phospholipase C)-binding, and rhodopsin-binding. We examined the in vivo splicing of dgq in adult flies and find that, in all but the male gonads, only two isoforms are expressed. One, dgqA, is the original visual isoform and is expressed in eyes, ocelli, brain, and male gonads. The other, dgqB, has the three novel exons and is widely expressed. Remarkably, all three nonvisual B exons are highly similar (82% identity at the amino acid level) to the Gq alpha family consensus, from Caenorhabditis elegans to human, but all three visual A exons are divergent (61% identity). Intriguingly, we have found a third isoform, dgqC, which is specifically and abundantly expressed in male gonads, and shares the divergent rhodopsin-binding exon of dgqA. We suggest that DGqC is a candidate for the light-signal transducer of a testes-autonomous photosensory clock. This proposal is supported by the finding that rhodopsin 2 and arrestin 1, two photoreceptor-cell-specific genes, are also expressed in male gonads. IMAGES:

DGq是异源三聚体GTP酶（heterotrimeric GTPase）的Gα亚基，在果蝇视觉通路中介导视紫红质（rhodopsin）与磷脂酶C（phospholipase C）的信号偶联。本研究通过在基因银行（GenBank）数据库中检索未被注释的编码序列，在dgq基因中发现了三个重复外显子。这些可变外显子所编码的结构位点分别参与GTP酶活性调控、Gβ结合、NorpA（磷脂酶C）结合以及视紫红质结合过程。我们对成年果蝇体内dgq基因的可变剪接模式进行了分析，结果显示，除雄性生殖腺外，其余组织仅表达两种同工型。其中dgqA为原始视觉型同工型，在眼、单眼、脑及雄性生殖腺中均有表达；dgqB则包含上述三个新型外显子，其表达谱更为广泛。值得注意的是，三个非视觉型B外显子与从秀丽隐杆线虫（Caenorhabditis elegans）到人类的Gqα家族保守序列具有高度同源性（氨基酸水平上的相似度达82%），而三个视觉型A外显子则呈现显著趋异（氨基酸相似度仅为61%）。更有趣的是，我们还发现了第三种同工型dgqC，其特异性且高表达于雄性生殖腺，且共享dgqA的趋异变视紫红质结合外显子。我们推测DGqC可能作为睾丸自主光感受生物钟的光信号转导因子。这一假说得到了以下研究结果的支持：视紫红质2（rhodopsin 2）与抑制蛋白1（arrestin 1）这两种感光细胞特异性基因同样在雄性生殖腺中表达。IMAGES: