Essential regulatory subunit of the mitochondrial calcium uniporter complex (uniplex), a complex that mediates calcium uptake into mitochondria (PubMed:24231807, PubMed:26774479, PubMed:27099988, PubMed:30454562, PubMed:31080062, PubMed:32315830, PubMed:32494073, PubMed:32762847, PubMed:32790952, PubMed:33296646). Required to bridge the calcium-sensing proteins MICU1 with the calcium-conducting subunit MCU (PubMed:24231807, PubMed:30454562, PubMed:32494073, PubMed:32762847, PubMed:32790952). Acts by mediating activation of MCU and retention of MICU1 to the MCU pore, in order to ensure tight regulation of the uniplex complex and appropriate responses to intracellular calcium signaling (PubMed:27099988, PubMed:31080062, PubMed:32315830, PubMed:33296646). {ECO:0000269|PubMed:24231807, ECO:0000269|PubMed:26774479, ECO:0000269|PubMed:27099988, ECO:0000269|PubMed:30454562, ECO:0000269|PubMed:31080062, ECO:0000269|PubMed:32315830, ECO:0000269|PubMed:32494073, ECO:0000269|PubMed:32762847, ECO:0000269|PubMed:32790952, ECO:0000269|PubMed:33296646}. This is the function of SMDT1 (single-pass membrane protein with aspartate rich tail 1, Ensembl gene identifier ENSG00000183172).