Abstract

Prokineticins 1 and 2 (PK1 and PK2) have been recently identified from humans and other mammals and play multiple functional roles. PK proteins are ligands for two G protein-coupled receptors, PK receptor 1 (PKR1) and PK receptor 2 (PKR2). Here, we report the molecular cloning and pharmacological characterization of an alternatively spliced product of the PK2 gene encoding 21 additional amino acids compared with PK2, designated PK2L (for PK2 long form). PK2L mRNA is broadly expressed, as is PK2. However, PK2L mRNA expression is lower in brain, undetectable in kidney, and much higher in lung and spleen than that of PK2. We expressed PK2L in mammalian cells and characterized the resulting peptide in comparison with PK1 and PK2. Biochemical characterization indicates that secreted PK2L protein is processed into a smaller peptide by proteolytic cleavage. We designate this smaller form of peptide as PK2β. Coexpression of furin with PK2L significantly increased the PK2β processing efficiency. Functional studies showed that PK1, PK2, and PK2β stimulate intracellular Ca²⁺ responses in PKR1-expressing cells with similar potencies. However, the PK2β stimulus of Ca²⁺ responses in PKR2-expressing cells is at least 10-fold less potent than that of PK1 or PK2. Differences in receptor selectivity combined with differential tissue expression patterns suggest PK2 and PK2β might have different functions in vivo. PKRs have been reported to couple to G_q and G_i proteins. In this report, we show that PKs not only stimulate Ca²⁺ mobilization but also induce cAMP accumulation in PKR-expressing cells.