Advances in Understanding Carboxysome Assembly in Prochlorococcus and Synechococcus Implicate CsoS2 as a Critical Component

Fei Cai; Zhicheng Dou; Susan L Bernstein; Ryan Leverenz; Eric B Williams; Sabine Heinhorst; Jessup Shively; Gordon C Cannon; Cheryl A Kerfeld

doi:10.3390/life5021141

Advances in Understanding Carboxysome Assembly in Prochlorococcus and Synechococcus Implicate CsoS2 as a Critical Component

Life (Basel). 2015 Mar 27;5(2):1141-71. doi: 10.3390/life5021141.

Authors

Fei Cai^{1

2}, Zhicheng Dou³, Susan L Bernstein^{4

5}, Ryan Leverenz⁶, Eric B Williams⁷, Sabine Heinhorst⁸, Jessup Shively⁹, Gordon C Cannon¹⁰, Cheryl A Kerfeld^{11

12

13}

Affiliations

¹ Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720, USA. fcai@lbl.gov.
² Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA. fcai@lbl.gov.
³ Department of Chemistry and Biochemistry, The University of Southern Mississippi, Hattiesburg, MS 39406-5043, USA. zdou@umich.edu.
⁴ Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720, USA. slbernstein@lbl.gov.
⁵ Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA. slbernstein@lbl.gov.
⁶ MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing, MI 48824, USA. rlleverenz@lbl.gov.
⁷ Department of Chemistry and Biochemistry, The University of Southern Mississippi, Hattiesburg, MS 39406-5043, USA. Eric.Williams@usm.edu.
⁸ Department of Chemistry and Biochemistry, The University of Southern Mississippi, Hattiesburg, MS 39406-5043, USA. sabine.heinhorst@usm.edu.
⁹ Department of Genetics and Biochemistry, Clemson University, Clemson, SC 29634, USA. shy53911@bellsouth.net.
¹⁰ Department of Chemistry and Biochemistry, The University of Southern Mississippi, Hattiesburg, MS 39406-5043, USA. Gordon.Cannon@usm.edu.
¹¹ Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720, USA. ckerfeld@lbl.gov.
¹² Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA. ckerfeld@lbl.gov.
¹³ MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing, MI 48824, USA. ckerfeld@lbl.gov.

Abstract

The marine Synechococcus and Prochlorococcus are the numerically dominant cyanobacteria in the ocean and important in global carbon fixation. They have evolved a CO2-concentrating-mechanism, of which the central component is the carboxysome, a self-assembling proteinaceous organelle. Two types of carboxysome, α and β, encapsulating form IA and form IB d-ribulose-1,5-bisphosphate carboxylase/oxygenase, respectively, differ in gene organization and associated proteins. In contrast to the β-carboxysome, the assembly process of the α-carboxysome is enigmatic. Moreover, an absolutely conserved α-carboxysome protein, CsoS2, is of unknown function and has proven recalcitrant to crystallization. Here, we present studies on the CsoS2 protein in three model organisms and show that CsoS2 is vital for α-carboxysome biogenesis. The primary structure of CsoS2 appears tripartite, composed of an N-terminal, middle (M)-, and C-terminal region. Repetitive motifs can be identified in the N- and M-regions. Multiple lines of evidence suggest CsoS2 is highly flexible, possibly an intrinsically disordered protein. Based on our results from bioinformatic, biophysical, genetic and biochemical approaches, including peptide array scanning for protein-protein interactions, we propose a model for CsoS2 function and its spatial location in the α-carboxysome. Analogies between the pathway for β-carboxysome biogenesis and our model for α-carboxysome assembly are discussed.