fsTnI

fsTnI, short for fast skeletal troponin I, is a protein isoform of the troponin I subunit found in skeletal muscle. It is a component of the troponin complex, which also includes troponin T (TnT) and troponin C (TnC). The complex regulates muscle contraction in response to intracellular calcium by controlling the position of tropomyosin on actin filaments. fsTnI is primarily expressed in fast-twitch (type II) skeletal muscle fibers and differs from slow skeletal troponin I (ssTnI) and cardiac troponin I (cTnI), which are encoded by different gene isoforms.

Gene and expression: In humans, fsTnI is encoded by the TNNI2 gene. Its expression is enriched in fast glycolytic fibers and, to a lesser extent, in other fiber types depending on developmental stage, training, and fiber-type composition. Alternative splicing can generate multiple fsTnI isoforms, contributing to variations in protein function among muscles.

Function and regulation: As part of the troponin complex, fsTnI binds to actin, TnC, and TnT to inhibit actin-myosin interaction at low calcium levels. When calcium binds to TnC, conformational changes relieve this inhibition, enabling cross-bridge cycling and muscle contraction. fsTnI influences the calcium sensitivity and contractile properties of fast-twitch fibers, contributing to their rapid contraction and relaxation dynamics.

Clinical and research context: Mutations in TNNI2 have been linked to skeletal muscle disorders in some studies, and fsTnI serves as a model for understanding fiber-type–specific regulation of contraction. It is studied using antibody-based detection, mass spectrometry, and gene expression techniques to explore muscle physiology and adaptation. See also troponin complex, fast- and slow-twitch troponin I, and TNNI2.