Final answer:
The nucleating complex for the initial nucleation of microfilaments and side branches is actin. Actin filaments are essential for many cellular processes, including muscle contraction, where the tropomyosin-troponin complex regulates the interaction between actin and myosin.
Step-by-step explanation:
The nucleating complex that is used both for the initial nucleation of MFs (microfilaments, also known as actin filaments) and for nucleation of side branches is actin. Actin is a protein that polymerizes to form microfilaments, which are a major component of the cytoskeleton in eukaryotic cells. These actin filaments are involved in various cellular activities, including muscle contraction, cell motility, and cell division. The tropomyosin-troponin complex plays a crucial role in muscle contraction by regulating the interaction between actin and myosin filaments. Tropomyosin binds along the length of the actin filament, and when calcium ions bind to troponin, this complex changes conformation, exposing the myosin-binding sites on actin and allowing for muscle contraction.