Burkart Christoph, Qiu Feng, Brendel Sigrun, Benes Vladimir, Hååg Petra, Labeit Siegfried, Leonard Kevin, Bullard Belinda
Institut für Anästhesiologie und Operative Intensivmedizin, Universitätsklinikum Mannheim, D-68167 Mannheim, Germany.
J Mol Biol. 2007 Apr 6;367(4):953-69. doi: 10.1016/j.jmb.2007.01.059. Epub 2007 Jan 27.
The passive elasticity of the sarcomere in striated muscle is determined by large modular proteins, such as titin in vertebrates. In insects, the function of titin is divided between two shorter proteins, projectin and sallimus (Sls), which are the products of different genes. The Drosophila sallimus (sls) gene codes for a protein of 2 MDa. The N-terminal half of the protein is largely made up of immunoglobulin (Ig) domains and unique sequence; the C-terminal half has two stretches of sequence similar to the elastic PEVK region of titin, and at the end of the molecule there is a region of tandem Ig and fibronectin domains. We have investigated splicing pathways of the sls gene and identified isoforms expressed in different muscle types, and at different stages of Drosophila development. The 5' half of sls codes for zormin and kettin; both proteins contain Ig domains and can be expressed as separate isoforms, or as larger proteins linked to sequence downstream. There are multiple splicing pathways between the kettin region of sls and sequence coding for the two PEVK regions. All the resulting protein isoforms have sequence derived from the 3' end of the sls gene. Splicing of exons varies at different stages of development. Kettin RNA is predominant in the embryo, and longer transcripts are expressed in larva, pupa and adult. Sls isoforms in the indirect flight muscle (IFM) are zormin, kettin and Sls(700), in which sequence derived from the end of the gene is spliced to kettin RNA. Zormin is in both M-line and Z-disc. Kettin and Sls(700) extend from the Z-disc to the ends of the thick filaments, though, Sls(700) is only in the myofibril core. These shorter isoforms would contribute to the high stiffness of IFM. Other muscles in the thorax and legs have longer Sls isoforms with varying amounts of PEVK sequence; all span the I-band to the ends of the thick filaments. In muscles with longer I-bands, the proportion of PEVK sequence would determine the extensibility of the sarcomere. Alternative Sls isoforms could regulate the stiffness of the many fibre types in Drosophila muscles.
横纹肌中肌节的被动弹性由大型模块化蛋白质决定，如脊椎动物中的肌联蛋白。在昆虫中，肌联蛋白的功能由两种较短的蛋白质分担，即肌动蛋白结合蛋白和沙利莫斯蛋白（Sls），它们是不同基因的产物。果蝇沙利莫斯蛋白（sls）基因编码一种2兆道尔顿的蛋白质。该蛋白质的N端一半主要由免疫球蛋白（Ig）结构域和独特序列组成；C端一半有两段与肌联蛋白的弹性PEVK区域相似的序列，在分子末端有一个串联Ig和纤连蛋白结构域的区域。我们研究了sls基因的剪接途径，并鉴定了在不同肌肉类型以及果蝇发育不同阶段表达的异构体。sls的5'端一半编码佐明蛋白和结蛋白；这两种蛋白质都含有Ig结构域，可以作为单独的异构体表达，也可以作为与下游序列相连的较大蛋白质表达。在结蛋白区域和编码两个PEVK区域的序列之间存在多种剪接途径。所有产生的蛋白质异构体都有来自sls基因3'端的序列。外显子的剪接在发育的不同阶段有所不同。结蛋白RNA在胚胎中占主导，较长的转录本在幼虫、蛹和成虫中表达。间接飞行肌（IFM）中的Sls异构体是佐明蛋白、结蛋白和Sls(700)，其中来自基因末端的序列被剪接到结蛋白RNA上。佐明蛋白存在于M线和Z盘。结蛋白和Sls(700)从Z盘延伸到粗肌丝末端，不过，Sls(700)仅存在于肌原纤维核心。这些较短的异构体将有助于IFM的高刚度。胸部和腿部其他肌肉有更长的Sls异构体，其PEVK序列数量不同；所有这些异构体都跨越I带至粗肌丝末端。在I带较长的肌肉中，PEVK序列的比例将决定肌节的伸展性。不同的Sls异构体可以调节果蝇肌肉中多种纤维类型的刚度。
