Caloxin2A 1 is a novel peptide that inhibits the activity of Plasma Membrane Calcium ATPase (PMCA). PMCA is known to play a role in homeostasis of cytosolic calcium and cell signaling. There are 4 genes (PMCA1-4) that code for the various isoforms of the calcium pump. Based on hydropathy plots, PMCA proteins have 5 putative extracellular domains. We screened combinatorial peptide phage display library for binding to specific extracellular targets. Caloxin 2A1 was obtained as a peptide sequence that would bind to the 2nd putative extracellular domain of PMCA 1 isoform. Caloxin2A1 selectively inhibited the Ca2+-Mg2+ ATPase activity in human erythrocyte leaky ghosts that express mainly PMCA 4 isoform. It produced 50% inhibition of the pump activity at 0.4 mM. Caloxin2A1 inhibited the formation of the acid stable 140 kDa acyl phosphate in the reaction cycle of the calcium pump in the human erythrocyte leaky ghosts. It also produced endothelium dependent relaxation in the pig coronary artery. The random peptide phage display library was screened again with higher stringency to obtain caloxin with higher affinity in order to be cost effective and with greater therapeutic potential. This time, the targets were the 2nd putative extracellular domain of PMCA 1 and 2nd and 3rd putative domains of PMCA 4. The peptides selected for binding to the 2nd putative extracellular domain of PMCA 4 selectively inhibited the Ca2^+-Mg^2+ ATPase activity in human erythrocyte leaky ghosts but with a similar affinity as Caloxin2A1. The peptide selected for binding to the 3rd putative extracellular domain of PMCA 4 was hydrophobic and water insoluble. Substitution of its C-terminus amino acid with lysine residue made the peptide water-soluble and it did inhibit the Ca^2 +-Mg^2 + ATPase with slightly higher affinity. However, the inhibition was due to hydrophobicity of the peptide as the randomized version of the peptide also produced inhibition. We have obtained the first selective inhibitor of PMCA and shown that perturbing extracellular targets can affect protein activity even though most of the functional groups of this protein are in the cytosol. / Thesis / Master of Science (MS)