Skeletal muscle is the key tissue used by animals for locomotion and other movements of the body. It thus has important implications in physiology and medicine. The contractile machinery consists of thick filaments, composed of myosin and myosin-binding proteins, thin filaments, composed of actin and the thin-filament associated proteins troponin and tropomyosin, Z-line proteins, and giant proteins involved in assembly of the filaments into the contractile units called sarcomeres [1
]. Within a single adult skeletal muscle, distinct muscle fiber types, with different sets of protein isoforms and different functional properties, can be found side by side [2
]. This heterogeneity enables a flexible contractile response. From studies of Drosophila, C. elegans
, bivalvia, decapod crustaceans, and other invertebrates, it is recognized that invertebrate muscle genes and proteins show numerous variations on the common theme of thick and thin filament assembly and interaction [3
]. These variations have played an important role in revealing common themes and how specialization allows animals to adapt to particular needs.
Tarantula skeletal muscle has emerged as a key model system for understanding the structural characteristics of muscle thick filaments. The thick filaments have a highly ordered array of myosin heads, which is easier to study structurally than that in the more complex and less stable filaments found in vertebrate skeletal muscle. Studies of tarantula thick filaments have led to a new understanding of thick filament assembly and regulation [4
]. These newly gained insights have been shown to be directly relevant to myosin function in vertebrate muscles. In addition, tarantula, like most invertebrates, is dually-regulated, via both actin-linked and myosin-linked systems [5
], and its thin filaments have provided insights into actin-myosin interaction and regulation [7
]. Despite these structural advances, sequence information on tarantula muscle proteins, which could provide a critical complement to structural and functional knowledge of this muscle, has been lacking: no tarantula muscle-related sequences, either mRNA or protein, are yet in public databases. Furthermore, the subphylum chelicerata to which the tarantula belongs has been infrequently sampled for muscle research, and there are only a few records of muscle proteins.
Expressed sequence tags (ESTs) are short single-pass sequence reads generated from either 5' or 3' end of cDNAs. They provide a quick and inexpensive route for discovering new genes and obtaining data on gene expression [8
]. To obtain a rough picture of the genes expressed in tarantula skeletal muscle, we constructed a non-normalized cDNA library from Aphonopelma
sp. leg muscle, and randomly generated and analyzed 2507 5'ESTs. We have identified transcripts of skeletal muscle-related genes, most with more than one isoform, and predicted full-length as well as partial cDNA sequences of major myofibrillar proteins based on bioinformatics analysis, pair-wise end sequencing, and primer walking. We also provide a preliminary analysis of the properties of the peptides deduced from the above transcripts.
We believe this to be the first report of an EST strategy applied to tarantula skeletal muscle. The sequence data should greatly facilitate further structural and functional research.