Molecular characterization, expression profile and polymorphisms of the porcine TNNC2 gene

摘要

The troponin complex, which regulates muscle contraction and relaxation, is located on the thin filament of striated muscle and acts as a calcium-sensitive switch. It is composed of three subunits: troponin C (TnC), the calcium binding subunit; troponin T (TnT), which is involved in the attachment of the complex to tropomyosin; and troponin I (TnI), the inhibitory subunit (Parmachek and Leiden 1991; Schiaffino et al. 1993). Multiple isoforms of each of these subunits have been identified, and they are expressed with different patterns of tissue specificity and developmental regulation. TnC plays a key role in the regulation of contraction, acting as a Ca2+ sensor to switch on the development of tension when the Ca2+ concentration increases. TnC is a dumbbell-shaped protein with two globular heads linked by a central α-helix (Farah and Reinach 1985). Each globular head is made up of two Ca2+ EF-hand domains: the two high-affinity carboxy-terminal sites III and IV that keep TnC bound to the thin filament (Zot and Potter 1982), and the two low-affinity carboxy-terminal sites I and II that regulate muscle contraction (Potter and Gergely 1975). TnC is composed of two structurally similar but distinct isoforms: slow skeletal troponin C (TNNC1) and fast skeletal troponin C (TNNC2). The latter has been immunocytochemically localized to fast skeletal muscle, while TNNC1 has been shown to be expressed in slow skeletal and cardiac muscle (Parmachek and Leiden 1991).It has been demonstrated that TNNC2 is the calcium-binding subunit of the myofibrillar thin filament that regulates excitation-contraction coupling. The porcine TNNC2 gene has been assigned to chromosome 17q2.1-q2.2 (Zambonelli et al. 2000), while the human TNNC2 gene is located on chromosome 20q12-q13.11 (Tiso et al. 1997). The 490 bp mRNA sequence of the porcine TNNC2 gene, which contains a complete ORF of 483 bp encoding a protein of 160 amino acids, has been cloned (Tian et al. 2006). The TNNC2 genes of vertebrates share a conserved six-exon gene structure (Parmachek et al. 1990; Tiso et al. 1997).In the livestock industry, the influence of genetics on meat quality is now well recognized, but still poorly understood. Domestic animals have been subjected to intensive selection since the beginning of domestication, particularly for traits related to growth, development, body composition and reproduction (Andersson and Georges 2004). The TNNC2 gene has been identified to be an excellent model system for studies of developmentally regulated gene expression in skeletal muscle (Parmachek et al. 1990). Therefore, in this study, we isolated the full-length cDNA of the porcine TNNC2 gene and characterized its genomic organization, mRNA expression, prokaryotic expression and polymorphism in order to further study its function in myofibrillar protein turnover and its association with pork quality.