What is driving the diversification of the N-terminal α1-helix of NRC helper NLRs?

Nucleotide-binding leucine-rich repeat (NLR) proteins are intracellular immune receptors that recognize and trigger immune responses to pathogens. Plant NLRs can function as single receptor units or in networks with functionally specialized sensor-helper configurations. Upon pathogen perception by upstream sensors, helper CC-type NLRs form a higher-order “resistosome” complex that presumably integrates into the plasma membrane as a calcium-permeable channel to initiate immune signaling. Approximately 20% of plant CC-NLRs carry a conserved N-terminal motif, termed as MADA, that defines an α1-helix funnel structure on the resistosome for channel activity and cell death induction. The MADA α1-helix is functionally conserved across angiosperms, as it can be swapped between phylogenetically unrelated CC-NLRs without affecting NLR activity. Additionally, the MADA α1-helix is crucial for resistosome function, as mutations and truncations in this region usually abolish its immune signaling function. Despite the conservation of the MADA motif, N-terminal regions of MADA-containing NLRs display diversity in sequence and length. The drivers of N-terminal diversification of MADA-CC-NLRs and the possible roles of the N-termini, besides calcium channel activity, are unknown. To study CC-NLR N-terminal sequence diversity and its molecular function, we focused on the well-characterized NRC clade of MADA-CC-NLRs that function as helpers in the Asterids NRC network. We curated a dataset of 1937 NRC proteins from 124 Solanales genomes and performed phylogenomic analyses on their N-termini to reconstruct their molecular evolution. Furthermore, we will evaluate the role of NRC N-termini using biochemistry, structural biology, and cell biology approaches. Our work provides insights into the evolution and molecular function of the N-terminal MADA-type α1-helix in NLRs.