Proteomic Analysis of an Unculturable Bacterial Endosymbiont (<i>Blochmannia</i>) Reveals High Abundance of Chaperonins and Biosynthetic Enzymes

Many insect groups have coevolved with bacterial endosymbionts
that live within specialized host cells. As a salient example, ants
in the tribe Camponotini rely on Blochmannia, an
intracellular bacterial mutualist that synthesizes amino acids and
recycles nitrogen for the host. We performed a shotgun, label-free,
LC/MS/MS quantitative proteomic analysis to investigate the proteome
of Blochmannia associated with Camponotus
chromaiodes. We identified more than 330 Blochmannia proteins, or 54% coverage of the predicted proteome, as well as
244 Camponotus proteins. Using the average intensity
of the top 3 “best flier” peptides along with spiking
of a surrogate standard at a known concentration, we estimated the
concentration (fmol/μg) of those proteins with confident identification.
The estimated dynamic range of Blochmannia protein
abundance spanned 3 orders of magnitude and covered diverse functional
categories, with particularly high representation of metabolism, information
transfer, and chaperones. GroEL, the most abundant protein, totaled
6% of Blochmannia protein abundance. Biosynthesis
of essential amino acids, fatty acids, and nucleotides, and sulfate
assimilation had disproportionately high coverage in the proteome,
further supporting a nutritional role of the symbiosis. This first
quantitative proteomic analysis of an ant endosymbiont illustrates
a promising approach to study the functional basis of intimate symbioses.