Bluetools partners at the Technion – Israel Institute of Technology, have identified and characterised a light-gated ion channel named HulaCCR1, discovered through metatranscriptomic analysis of a unicellular eukaryotic community in Lake Hula, Israel. The findings were published in the Journal of Molecular Biology.
HulaCCR1 belongs to a family of bacteriorhodopsin-like cation channelrhodopsins (BCCRs). The study notes that none of the organisms in the clade to which HulaCCR1 belongs have been cultured, yet the gene could be identified, cloned, and studied in detail.
The protein features a unique ETD motif in its third transmembrane helix, differing from the typical DTD motif found in other BCCRs. This difference became a key focus of the study, particularly regarding its role in spectral tuning and channel gating.
The research team expressed HulaCCR1 in mammalian cells and performed electrophysiological experiments, confirming that it functions as a non-specific cation channel activated by green light. The protein showed ion selectivity for monovalent cations, including sodium, and low selectivity for protons. It did not conduct anions. Mutational analysis revealed that the glutamate residue in the ETD motif plays a significant role in the protein’s spectral properties and gating kinetics.
Additionally, HulaCCR1 has long N/C-terminal extensions not typically seen in other BCCRs. The N-terminal extension includes a signal peptide, which was shown to be essential for efficient membrane localisation. Truncating this region impaired the protein’s ability to reach the cell membrane. Interestingly, while the C-terminal region appeared less essential, certain truncations affected membrane targeting, suggesting a supporting role in protein stability in the membrane.
In optogenetic experiments, HulaCCR1 successfully triggered neuronal firing in cultured rat hippocampal neurons, demonstrating its ability to modulate activity with light. It was expressed throughout the neuron, including axons, and responded reliably to both continuous and pulsed illumination.
This study expands the known diversity of channelrhodopsins and shows that metatranscriptomic data can be used not only to discover new proteins but also to understand their function without requiring cultured organisms. The discovery of HulaCCR1 highlights the potential of environmental sequencing for revealing new bioactive molecules in natural microbial communities, a foundational goal of the Bluetools project.