Binds as a heterodimer with protein S6 to the central domain of the 16S rRNA, where it helps stabilize the platform of the 30S subunit. Part of the 30S ribosomal subunit (PubMed:776663, PubMed:7556101, PubMed:10094780, PubMed:12244297, PubMed:12809609, PubMed:16272117, PubMed:27934701, PubMed:27906160, PubMed:27906161). Forms a tight heterodimer with protein S6. Belongs to the bacterial ribosomal protein bS18 family.