MYL4

MYL4 Full Name

myosin, light chain 4, alkali; atrial, embryonic

MYL4 Introduction

Myosin light chain 4, abbreviated as MYL4, is a protein-coding gene situated on human chromosome 17q21.32, a region known for harboring several sarcomeric gene clusters. It belongs to the EF-hand superfamily of calcium-binding proteins and is specifically classified as an alkali myosin light chain, distinct from the regulatory light chains that modulate phosphorylation-dependent contraction. The gene produces multiple transcript variants through alternative splicing, though the predominant isoform encodes a 195-amino-acid polypeptide with a molecular weight of approximately 22 kDa. MYL4 is evolutionarily related to the MYL1 and MYL3 genes, yet it possesses unique expression kinetics that reflect its specialized roles during cardiogenesis and in mature atrial tissue.

Figure 1. MYL4 (Myosin, light chain 4, alkali; atrial, embryonic): Gene, Structure, and Function.

Molecular Topology and Calcium-Binding Properties
The MYL4 protein consists of a conserved N-terminal region followed by three EF-hand helical motifs, each comprising a helix-loop-helix configuration that coordinates a single calcium ion with high specificity. This structural arrangement allows MYL4 to undergo conformational changes upon calcium occupancy, thereby influencing its interaction with the myosin heavy chain head domain. Unlike regulatory light chains, MYL4 does not directly control muscle contraction through phosphorylation but rather stabilizes the lever arm of the myosin head during the power stroke cycle. The protein's isoelectric point and surface charge distribution favor its incorporation into nascent sarcomeres, particularly during early developmental windows when rapid myofibrillogenesis occurs. Mutations within the calcium-binding loops have been shown to disrupt this structural integrity.

Temporal and Spatial Expression Profiles
MYL4 exhibits a highly restricted and developmentally regulated expression pattern, being predominantly transcribed in embryonic cardiac tissue and persisting into adulthood exclusively within the atria. During mammalian embryogenesis, MYL4 is one of the earliest myosin light chain isoforms to appear in the developing heart tube, coinciding with the initiation of rhythmic contractions around embryonic day 8.5 in mice. In postnatal life, its expression becomes progressively downregulated in ventricular chambers while remaining robustly maintained in atrial cardiomyocytes, where it coexists with the slower MYL3 isoform. This chamber-specific distribution is governed by combinatorial actions of transcription factors including GATA4, NKX2-5, and TBX5. Pathological stress or cardiac injury can reactivate MYL4 expression in ventricles, indicating a reversion to fetal gene programming.

Alternate Names for MYL4

MYL4; myosin, light chain 4, alkali; atrial, embryonic; myosin, light polypeptide 4, alkali; atrial, embryonic; myosin light chain 4; ALC1; AMLC; GT1; myosin