Mechanistic Insight into the Conformational Changes of Cas8 upon Binding
to Different PAM Sequences in the Transposon-Encoded Type I-F CRISPR-Cas
System
Abstract
The INTEGRATE system is a gene-editing approach that offers advantages
over the widely used CRISPR-Cas9 system. It does not introduce double
strand breaks in the target DNA but rather integrates the desired DNA
sequence directly into it. The first step in the integration process is
PAM recognition, which is critical to understanding and optimizing the
system. Experimental testing revealed varying integration efficiencies
of different PAM mutants, and computational simulations were carried out
to gain mechanistic insight into the conformational changes of Cas8
during PAM recognition. Our results showed that the interaction between
Arg246 and Guanine at position (-1) of the target strand is critical for
PAM recognition. We found that unfavorable interactions in the 5’-AC-3’
PAM mutant disrupted this interaction and may be responsible for its 0%
integration efficiency. Additionally, we discovered that PAM sequences
not only initiate the integration process but also regulate it through
an allosteric mechanism that connects the N-terminal domain and the
helical bundle of Cas8. This allosteric regulation was present in all
PAMs tested, even those with lower integration efficiencies, such as
5’-TC-3’ and 5’-AC-3’. We identified the Cas8 residues that are involved
in this regulation. Our findings provide valuable insights into PAM
recognition mechanisms in the INTEGRATE system and can help improve the
gene-editing technology.