melittin Protein in bee venom responsiblefor causing cell lysis after injection of bee-sting. bee premelittin Precursor to melittin. Premelittin has 22 additional amino acids at its N terminus which are later removed to release active venom protein. bee nitrogenase An enzyme involved in the fixation of nitrogen which occurs in bacteria. bacteria actin Protein found in muscles. A contractile protein. Structure like a bead. vertebrates myosin Protein found in muscles. A contractile protein. Structure like a thick filament. vertebrates hemoglobin Protein containing heme group required for transportation of oxygen through blood stream vertebrates myoglobin Protein containing heme group required for storage of oxygen in muscle vertebrates keratin A tough, water-insoluble protein used to make skin, hair, and nails mammals melanin A brownish-black pigment that aids in protecting the body against ultraviolet radiation mammals collagen Protein used to make ligaments and bones mammals cyclin A regulatory protein whose abundance varies during the cell cycle and which regulates biochemical events in a cell cycle-specific manner. eukaryotes and prokaryotes endonuclease An enzyme that breaks phosphodiester bonds within a nucleic acid molecule eukaryotes and prokaryotes exportin A protein involved in transport of molecules out of the nucleus. eukaryotes and prokaryotes helicase An enzyme that breaks base pairs in a double-stranded DNA molecule. eukaryotes and prokaryotes phosphofructokinase A key regulatory enzyme in glycolysis. The enzyme catalyzes one of the early steps in this pathway. eukaryotes and prokaryotes calmodulin A ubiquitous calcium-binding protein of 148 amino acid residues that is involved in a range of calcium-dependent signaling pathways. eukaryotes and prokaryotes gamma crystallins Monomeric proteins with a polypeptide chain of around 170 amino acid residues. Found in cells of the lens. eukaryotes retinol-binding protein Protein responsible for transporting the lipid alcohol Vitamin A (retinol) from its storage site in the liver to the various vitamin-A-dependent tissues. eukaryotes