<scp>STIMATE</scp> ‐Mustn1 Fusion Protein Prevents Ca ²⁺ Overload in Cells That Present Increased <scp>SOCE</scp>

Store operated Ca²⁺ entry (SOCE) is altered in neonatal platelets with respect to adult platelets. Here, we aim to analyze the changes in STIMATE translation that leads to generating STIMATE-mustn1 and its possible involvement in SOCE in neonatal platelets. As a result, we have found different expression patterns of STIMATE, mustn1 and STIMATE-mustn1 in platelets isolated from mothers with respect to their newborns. Calcium (Ca²⁺) experiments using MEG-01 cells overexpressing STIMATE or the fused protein STIMATE-mustn1 reveal that mustn1 binding to STIMATE impairs its role in SOCE; meanwhile, overexpression of mustn1 alone does not alter SOCE. Confocal images of MEG-01 and HEK293 cells reveal that the alteration in the expression of STIMATE modifies the co-localization between Orai1 and STIM1. Furthermore, STIMATE or STIMATE-mustn1 overexpression exhibits different intracellular location patterns, which were also different when compared between MEG-01 and HEK293 cells. Finally, STIMATE overexpression arrests the cell cycle of MEG-01 cells in the G2 phase, which represents a key step during the maturation of MEG-01 cells, but STIMATE-mustn1 overexpression does not change the cell cycle of MEG-01. Summarizing, we propose that overexpression of STIMATE-mustn1 by neonatal platelets might be an attempt to limit the exacerbated activation of SOCE found in these platelets.