May play a role during erythropoiesis through regulation of transcription factor DDIT3. Component of the small ribosomal subunit. Mature ribosomes consist of a small (40S) and a large (60S) subunit. The 40S subunit contains about 33 different proteins and 1 molecule of RNA (18S). The 60S subunit contains about 49 different proteins and 3 molecules of RNA (28S, 5.8S and 5S). Identified in a IGF2BP1-dependent mRNP granule complex containing untranslated mRNAs. Binds with high affinity to IPO4. Interacts with DDIT3. Localized in cytoplasmic mRNP granules containing untranslated mRNAs. The protein designated S3b has the same amino acid sequence as S3a except that it lacks the C-terminal 12 residues. It is probable that S3a is converted by proteolysis, either physiologically or fortuitously, to S3b. ADP-ribosylated at Tyr-155 by PARP1 in presence of HPF1. Belongs to the eukaryotic ribosomal protein eS1 family.