This study investigates the structural basis for dimerization of the paramyxovirus polymerase complex, specifically focusing on human parainfluenza virus type 3 (hPIV3). The researchers determined a 2.7 Å cryo-EM structure of the hPIV3 large protein (L) and phosphoprotein (P) complex, revealing detailed atomic features of the active site and distinct conformation of hPIV3 L with a unique β-strand latch. The study also provides insights into the structural basis of L-L dimerization, with the connector domain (CD) of a second L protein located at the putative template entry of the adjoining L. Functional studies show that disrupting the L-L interface causes a defect in RNA replication, which can be overcome by complementation, confirming the necessity of L dimerization for hPIV3 genome replication. These findings enhance our understanding of the structure-function properties of nsNSV polymerases and suggest new avenues for drug design.This study investigates the structural basis for dimerization of the paramyxovirus polymerase complex, specifically focusing on human parainfluenza virus type 3 (hPIV3). The researchers determined a 2.7 Å cryo-EM structure of the hPIV3 large protein (L) and phosphoprotein (P) complex, revealing detailed atomic features of the active site and distinct conformation of hPIV3 L with a unique β-strand latch. The study also provides insights into the structural basis of L-L dimerization, with the connector domain (CD) of a second L protein located at the putative template entry of the adjoining L. Functional studies show that disrupting the L-L interface causes a defect in RNA replication, which can be overcome by complementation, confirming the necessity of L dimerization for hPIV3 genome replication. These findings enhance our understanding of the structure-function properties of nsNSV polymerases and suggest new avenues for drug design.