Cryo-EM structure of the type III-E CRISPR-Cas effector gRAMP in complex with TPR-CHAT

Abstract

The type III-E CRISPR-Cas effector protein, named gRAMP, is the largest single-unit CRISPR-Cas effector. A caspase-like peptidase (TPR-CHAT) gene often co-occurs with gRAMP gene clusters. However, the exact mechanism of the recognition and cleavage of target RNA of Sb-gRAMP, as well as the molecular architecture of the CRISPR-guided caspase complex, remains unclear. Here, we report the cryo-EM structures of the type III-E effector Sb-gRAMP-crRNA in a complex with TPR-CHAT, with and without target ssRNA at 3.0 and 2.9 [A], respectively. The overall structure of the gRAMP-crRNA-ssRNA-TPR-CHAT complex adopts an "L"-shaped conformation, consisting of a copy of gRAMP, a copy of TPR-CHAT, a 37-nt crRNA, and an 18-nt target ssRNA. The data presented in this manuscript reveal the mechanism of recognition of crRNA and target ssRNA by gRAMP, also, this work reports the structure of the CRISPR type III-E effector in complex with the binding partner TPR-CHAT, which provides vital clues for elucidating the functional relation between the CRISPR-Cas system and caspase peptidase.