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Lanthipeptide synthetases are present in all domains of life. They catalyze a crucial step during lanthipeptide biosynthesis by introducing thioether linkages during posttranslational peptide modification. Lanthipeptides have a wide range of functions, including antimicrobial and morphogenetic activities. Intriguingly, several <i>Clostridium</i> species contain lanthipeptide synthetase-like genes of the class II (<i>lanM</i>) family but lack other components of the lanthipeptide biosynthetic machinery. In all instances, these genes are located immediately downstream of putative <i>agr</i> quorum sensing operons. The physiological role and mode of action of the encoded LanM-like proteins remain uncertain as they lack conserved catalytic residues. Here we show for the industrial organism <i>Clostridium acetobutylicum</i> that the LanM-like protein CA_C0082 is not required for the production of active AgrD-derived signaling peptide but nevertheless acts as an effector of Agr quorum sensing. Expression of CA_C0082 was shown to be controlled by the Agr system and is a prerequisite for granulose (storage polymer) formation. The accumulation of granulose, in turn, was shown to be required for maximal spore formation but also to reduce early solvent formation. CA_C0082 and its putative homologs appear to be closely associated with Agr systems predicted to employ signaling peptides with six-membered ring structures and may represent a new subfamily of LanM-like proteins. This is the first time their contribution to bacterial Agr signaling has been described.