Understanding of Ca 2؉ signaling requires the knowledge of proteins involved in this process. Among these proteins are sarco/endoplasmic reticulum Ca 2؉ -ATPases (SERCAs) that pump Ca 2؉ into the endoplasmic reticulum (ER). Recently, the human SERCA3 gene was shown to give rise to five isoforms (SERCA3a-e (h3a-h3e)). Here we demonstrate the existence of an additional new member, termed SERCA3f (h3f). By reverse transcriptase-PCR using monocytic U937 cell RNA, h3f mRNA was found to exclude the antepenultimate exon 21. h3f mRNA expression appeared as a human-specific splice variant. It was not found in rats or mice. h3f mRNA gave rise to an h3f protein differing in its C terminus from h3a-h3e. Of particular interest, h3f diverged in the first amino acids after the first splice site but presented the same last 21 amino acids as h3b. Consequently, we further investigated the structure-function-location relationships of the h3b and h3f isoforms. and (ii) the low apparent Ca 2؉ affinity and the enhanced rate of dephosphorylation of the E 2 P phosphoenzyme intermediate. Subcellular location of h3b and h3f by immunofluorescence and/or confocal microscopy using the h3b-and h3f-specific polyclonal and the pan-h3 monoclonal (PL/ IM430) antibodies suggested overlapping but distinct ER location. The endogenous expression of h3f protein was also proved in U937 cells. Altogether these data suggest that the SERCA3 isoforms have a more widespread role in cellular Ca 2؉ signaling than previously appreciated.Cell Ca 2ϩ signaling is a dynamic oscillatory process regulating a variety of important cellular functions such as secretion, contraction, metabolism, neuronal plasticity, and gene transcription (1). Accordingly, Ca 2ϩ signaling is strictly controlled in space, time, and amplitude. This very tight control is necessary to enable cells to extract relevant information from the Ca 2ϩ signal but also implies that disturbances in the intracellular Ca 2ϩ signaling can lead to a plethora of consequences.Understanding Ca 2ϩ signaling requires the basic knowledge of structures involved in this process. Among these structures are the Ca 2ϩ transport proteins inserted in the membranes of the endoplasmic reticulum (ER), 1 the intracellular reservoir of Ca 2ϩ ions. These include Ca 2ϩ channels (ryanodine and inositol 1,4,5-trisphosphate receptor channels) and Ca 2ϩ pumps (sarco/endoplasmic reticulum Ca 2ϩ -ATPases (SERCAs)). Although much data are available on the role of the channels in the elaboration of the Ca 2ϩ signal, little is known concerning how the signal may be modified by expression of SERCAs with different functional properties.In recent years, enormous advances in our understanding of SERCA structure-function relationships were made, including new insights regarding their three-dimensional structure (2, 3). Furthermore, relevant physiological functions of SERCAs have been defined through their potential role in various human diseases (4-6). However, some of the results contrast with those obtained in mice using SERCA1-3 knock ...