Accurately modeling the structures of proteins and their complexes using artificial intelligence is revolutionizing molecular biology. Experimental data enable a candidate-based approach to systematically model novel protein assemblies. Here, we use a combination of in-cell crosslinking mass spectrometry, co-fractionation mass spectrometry and the SubtiWiki database to identify protein-protein interactions in the model Gram-positive bacterium Bacillus subtilis. Pairing this with structure prediction by AIphaFold-Multimer, we identify novel interactors of central machineries that include the ribosome, RNA polymerase and pyruvate dehydrogenase, as well as interactions involving uncharacterized proteins, which we functionally validate. After controlling for the false-positive rate of the AlphaFold approach, we propose novel structural models of 153 dimeric and 14 trimeric protein assemblies. We show that crosslinking MS data can independently validate AlphaFold predictions in situ. Our approach uncovers protein-protein interactions inside cells, provides structural insight into their interaction interface, and is applicable to genetically intractable organisms, including pathogenic bacteria.