Final answer:
Smooth muscle features calmodulin instead of troponin, which, upon binding with calcium ions, activates myosin light chain kinase to enable muscle contraction.
Step-by-step explanation:
Smooth muscle has calmodulin and not troponin. In smooth muscle cells, the process of cross-bridge formation and muscle contraction is regulated not by the troponin-tropomyosin complex but by the protein calmodulin. When calcium ions enter the smooth muscle cell, they bind to calmodulin.
This calcium-calmodulin complex then activates the enzyme myosin light chain kinase, leading to the phosphorylation and activation of the myosin heads. The activated myosin heads can attach to actin-binding sites, which allows for the contraction of the smooth muscle fiber.