actin

A globular structural protein that polymerizes in a helical fashion to form an actin filament (or microfilament).

2012, Elaine N. Marieb & Katja Hoehn, Human Anatomy & Physiology, 8th ed., chapter 9 "Muscles and Muscle Tissue", page 281. Actin has kidney-shaped polypeptide subunits, called globular actin or G actin, which bear the active sites to which the myosin heads attach during contraction.

One of the six isoforms of actin.