A soluble 3D LC/MS/MS proteome of the filamentous cyanobacterium Nostoc punctiforme

Nostoc punctiforme is an oxygenic photoautotrophic cyanobacterium with multiple developmental states, which can form nitrogen-fixing symbioses with a variety of terrestrial plants. 3D LC/MS/MS shotgun peptide sequencing was used to analyze the proteome when N. punctiforme is grown in continuous moderate light with ammonia as the nitrogen source. The soluble proteome includes 1575 proteins, 50% of which can be assigned to core metabolic and transport functions. Another 39% are assigned to proteins with no known function, a substantially higher fraction than in the Escherichia coli proteome. Many expressed proteins protect against oxidative and light stress. Seventy-one sensor histidine kinases, response regulators, and serine/threonine kinases, individually and as hybrid, multidomain proteins, were identified, reflecting a substantial capacity to sense and respond to environmental change. Proteins encoded by each of the five N. punctiforme plasmids were identified, as were 10 transposases, reflecting the plasticity of the N. punctiforme genome. This core proteome sets the stage for comparison with that of other developmental states.