Spindle ATPase

Smooth muscle differs from skeletal muscle in various ways. Smooth muscles—which are found, for example, in blood vessel walls and in the walls of the intestines—do not contain any muscle fibers. In smooth-muscle cells, which are usually spindle-shaped, the contractile proteins are arranged in a less regular pattern than in striated muscle. Contraction in this type of muscle is usually not stimulated by nerve impulses, but occurs in a largely spontaneous way. Ca (in the form of Ca -calmodulin see p.386) also activates contraction in smooth muscle in this case, however, it does not affect troponin, but activates a protein kinase that phosphorylates the light chains in myosin and thereby increases myosin s ATPase activity. Hormones such as epinephrine and angiotensin II (see p. 330) are able to influence vascular tonicity in this way, for example. 

Several lines of evidence indicate that MPF-catalyzed phosphorylation also plays a role in chromosome condensation and formation of the mitotic spindle apparatus. For instance, genetic experiments in the budding yeast S. cerevisiae identified a family of SMC (structural maintenance of chromosomes) proteins that are required for normal chromosome segregation. These large proteins (-1200 amino acids) contain characteristic ATPase domains at their C-termlnus and long regions predicted to participate in colled-coll structures. 

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