Interaction of caldesmon with endoplasmic reticulum membrane: effects on the mobility of phospholipids in the membrane and on the phosphatidylserine base-exchange reaction

Intrinsically disordered caldesmon binds calmodulin via the "buttons on a string" mechanism

We show here that chicken gizzard caldesmon (CaD) and its C-terminal domain (residues 636–771, CaD₁₃₆) are intrinsically disordered proteins. The computational and experimental analyses of the wild type CaD₁₃₆ and series of its single tryptophan mutants (W674A, W707A, and W737A) and a double tryptophan mutant (W674A/W707A) suggested that although the interaction of CaD₁₃₆ with calmodulin (CaM) can be driven by the non-specific electrostatic attraction between these oppositely charged molecules, the specificity of CaD₁₃₆-CaM binding is likely to be determined by the specific packing of important CaD₁₃₆ tryptophan residues at the CaD₁₃₆-CaM interface. It is suggested that this interaction can be described as the “buttons on a charged string” model, where the electrostatic attraction between the intrinsically disordered CaD₁₃₆ and the CaM is solidified in a “snapping buttons” manner by specific packing of the CaD₁₃₆ “pliable buttons” (which are the short segments of fluctuating local structure condensed around the tryptophan residues) at the CaD₁₃₆-CaM interface. Our data also show that all three “buttons” are important for binding, since mutation of any of the tryptophans affects CaD₁₃₆-CaM binding and since CaD₁₃₆ remains CaM-buttoned even when two of the three tryptophans are mutated to alanines.

Postnatal changes in caldesmon expression and localization in cardiac myocytes

Caldesmon is a heat-stable protein found in both muscle and non-muscle tissue. It binds to a number of contractile and cytoskeletal proteins and may be involved in regulating acto-myosin interaction in smooth muscle cells and/or the assembly of microfilaments in muscle and non-muscle cells. We have shown previously that caldesmon is localized at the Z-lines in adult cardiac myocytes and that both the low- and high-molecular-weight forms (/-caldesmon and h-caldesmon, respectively) are present in atrial and ventricular myocytes. Here we examined the expression of caldesmon and its localization in freshly isolated cardiac myocytes during postnatal development and when these myocytes were grown in culture. We found that /-caldesmon is expressed in both neonatal and adult rat ventricular myocytes. The expression of h-caldesmon, however, was not detected in myocytes from newborn animals but increased during the first 2 weeks of postnatal development. Caldesmon was generally not co-localized with alpha-actinin at the Z-lines in neonatal myocytes but became increasingly more so during the first 2 weeks of postnatal development. When myocytes from 5- and 10-day-old rats were grown in primary culture, h-caldesmon expression decreased and caldesmon could not be detected at the Z-lines in the cultured cells. These results indicate that caldesmon plays a role at the Z-lines in adult cardiac myocytes; however, its localization at the Z-lines is not necessary for the prenatal development that occurs at these sites or for the establishment of a contractile phenotype in cultured cardiac myocytes.

Biogenesis and cellular dynamics of aminoglycerophospholipids

Aminoglycerophospholipids phosphatidylserine (PtdSer), phosphatidylethanolamine (PtdEtn), and phosphatidylcholine (PtdCho) comprise about 80% of total cellular phospholipids in most cell types. While the major function of PtdCho in eukaryotes and PtdEtn in prokaryotes is that of bulk membrane lipids, PtdSer is a minor component and appears to play a more specialized role in the plasma membrane of eukaryotes, e.g., in cell recognition processes. All three aminoglycerophospholipid classes are essential in mammals, whereas prokaryotes and lower eukaryotes such as yeast appear to be more flexible regarding their aminoglycerophospholipid requirement. Since different subcellular compartments of eukaryotes, namely the endoplasmic reticulum and mitochondria, contribute to the biosynthetic sequence of aminoglycerophospholipid formation, intracellular transport, sorting, and specific function of these lipids in different organelles are of special interest.