Gen. Physiol. Biophys. (1988), 7, 95—107 95 Interactions of Calcium Binding Proteins, Parvalbumin and a-lactalbumin, with Dipalmitoylphosphatidylcholine Vesicles E. A. PERMYAKOV, D. I. KREIMER, L. P. KALINICHENKO and V. L. SHNYROV Institute of Biological Physics, Academy of Sciences of the USSR, 142292 Pushchino, Moscow Region, USSR Abstract. Interactions of Ca 2+ binding proteins, pike (Esox lucius) parvalbumins pi 4.2 and 5.0, and bovine and human a-lactalbumins, with dipalmitoylpho- sphatidylcholine vesicles were studied by means of scanning microcalorimetry and intrinsic tyrosine and tryptophan fluorescence methods. The interactions of pike parvalbumins are modulated by Ca 2 + and Mg 2+ binding to the protein and induce some changes in the physical properties of both the proteins and lip- osomes. Liposomes increased thermal stability of Ca 2+ -loaded parvalbumin and decreased thermal stability of both Mg 2+ -loaded and metal- -free protein. The interaction of parvalbumin with liposomes affects the phase transition from gel to liquid-crystalline state in liposomes. Ca 2+ -loaded a-lac- talbumin interacts with liposomes in its native state while the metal-free protein binds to the liposomes mainly in its thermally denatured state. The results of the microcalorimetric and spectrofluorometric studies are supported by data ob- tained by means of gel-chromatography on Sepharose 4B. It may be suggested that these metal-modulated interactions of Ca 2+ -binding proteins with mem- branes have some functional significance. Key words: Parvalbumins — a-lactalbumins — Liposomes — Interaction-Ca 2+ and Mg 2+ binding — Fluorescence-microcalorimetry Introduction Calcium binding proteins play an important role in cell regulation. The regulato- ry functions of proteins such as calmodulin or troponin C is well known. However, there are other calcium binding proteins with hither to unclear physiological functions. The group of these proteins includes parvalbumins and a-lactalbumins. Parvalbumins are a family of low molecular mass proteins (11.000— 13.000). The parvalbumin molecule consists of six helical regions (A to F)