Regulation of αIIbβ3-Signaling by Myomesin Family of Proteins.

Integrins are the type-I transmembrane proteins. They function as heterodimers of α and β subunits. The platelet integrin αIIbβ3 (GPIIb-IIIa) served as the model system in understanding the signaling across integrin receptors. αIIbβ3 exists in a resting state on circulating platelets and can be activated via several inside-out signaling pathways resulting in binding to its ligands, such as fibrinogen (Fg). The inside-out signaling appears to be regulated by several cytoskeletal and signaling proteins, which directly associate with the cytoplasmic tails of αIIbβ3. We have previously identified skelemin/myomesin-I as the cytoskeletal protein that interacted with the cytoplasmic tail of β3-integrins. Skelemin/myomesin-I is very closely related to two other members: myomesins-II and III. Myomesins I-III, along with other proteins such as titin and myosin light chain kinases, are members of a much larger superfamily of proteins. The hallmark of this family of proteins is the presence of a variable number of fibronectin (Fn)-like and immunoglobulin (Ig)-like motifs. Myomesin-I contains five Fn-motifs flanked by a total of seven Ig-like motifs. The interaction between β3-integrin and myomesin-I occurs via the C-terminal Ig motifs 3–7 of myomsin-I. In order to understand the functional consequence of this interaction between myomesin-I and β3-integrin, we measured the ligand binding properties of αIIbβ3 in a CHO cell model system. We found that expression of Ig motifs 3–7 increased Fg binding to the αIIbβ3, comparable to talin-head domain, used as the positive control. However, the expression of smaller and individual Ig motifs 4 and 5 resulted in still significantly higher Fg-binding to αIIbβ3. Ig motifs from myomesin-II also behaved in a similar fashion. We also found that myomesin-II is ubiquitously expressed in all the cell types tested, including platelets, suggesting myomesin-II is the likely candidate among various myomesin family members for the regulation of signaling across αIIbβ3. Our results suggest that the Ig motifs of the myomesin represent novel domains, which can regulate inside-out signaling across αIIbβ3.