Wednesday, October 15, 2008

Bacteriophages SPP1 contains two tail tube proteins produced by programmed translational frameshift

Origin and function of the two major tail proteins of bacteriophage SPP1. Auzat et al., Molecular Microbiology (2008) 70, 557-569

Bacteriophages are viruses that infect bacteria and are known to play roles in horizontal gene transfer. The majority of known bacteriophages have a head and long non-contractile tail that serves as a pipeline for phage genome delivery into bacterial cell. Here, the authors report that the tail tube of Bacillus subtilus bacteriaphage SPP1 is comprised of two proteins, gp17.1 and gp17.1* , that are produced by a translational frameshift. This mosaic construction of tail tube was found to be important for assembly of the functional tail tube, but its significance is not fully uncovered yet.

Previously, it was shown that the key event of phage DNA injection in bacterial cell is a rearrangement of the inner wall of the tail tube (EMBO J, (2007) 26, 3720-3728). In this article, the authors separated SSP1 tail proteins by SDS-PAGE and found a protein band which was not expected from phage DNA sequence. Protein sequencing analysis indicated that the unexpected protein designated gp17.1* has the same amino-terminal sequence as that of tail protein gp17.1. gp17.1* is 10 kDa larger than gp17.1. The tail tube is made up of these two proteins at the ratio of 1:3. Based on the molecular size of gp17.1* and the sequences of gene 17.1, authors postulated that 17.1* is produced by a translational frameshift. Using site-directed mutagenesis of coding sequence 17.1 with protein profile analysis of the mutant phages, authors found out that 5'-CCCUAA-3' sequence located at the end of coding sequence 17.1 was the frameshift position.

To get insight into the function of gp17.1*, the authors constructed mutant phages that have tail tubes comprised exclusively of gp17.1 or gp17.1* and analyzed their structures by electron microscopy. When phages are assembled under the condition which either gp17.1 or gp17.1* are exclusively expressed, significant numbers of tailless heads (capsids) are made. This suggests the 3:1 ratio of gp17.1 and gp17.1* is important for correct phage assembly. Interestingly, both mutant phages had infection activity. The length and flexibility of mutant tails composed of either gp17.1 or gp17.1* were identical to SSP1 wild-type tails. gp17.1*-specific tail has protrusions on surface while gp17.1-specific tail has smooth surface. Authors postulate that carboxyl-terminus of gp17.1* causes protrusions on tail surface which facilitate initial contact of phages and attachment to the bacterial surface, while gp17.1 is ensures correct assembly of tail tube.

The potential translational frameshift site producing a carboxyl-terminus extension in a protein are also found in other phage surface protein genes (Mol Microbiol. (2003) 50, 303-317). Why do phages need two types of surface proteins which are identical except the carboxyl end extension of one of the proteins? What is the significance of strict ratio of the two proteins? Why is this the best strategy for phages? What is the target of the protrusion on tail tube surface? These are still interesting mysteries. It might be interesting to compare the host ranges of mutant phages that have either one of the two surface proteins.

Origin and Function of the Two Major Tail Proteins of Bacteriophage SPP1. Auzat I, Dröge A, Weise F, Lurz R, Tavares P. Molecular Microbiology (2008) 70: 557-569

Structure of bacteriophage SPP1 tail reveals trigger for DNA ejection. Plisson C, White HE, Auzat I, Zafarani A, São-José C, Lhuillier S, Tavares P, Orlova EV,
EMBO J (2007) 26:3720-8.

Genome and proteome of Listeria monocytogenes phage PSA: an unusual case for programmed + 1 translational frameshifting in structural protein synthesis. Zimmer M, Sattelberger E, Inman RB, Calendar R, Loessner MJ., Molecular Microbiology. (2003) 50:303-317.

Hirokazu Yano (university of idaho)

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