The Two AGPase Subunits Evolve at Different Rates in Angiosperms, yet They Are Equally Sensitive to Activity-Altering Amino Acid Changes When Expressed in Bacteria

Abstract

The rate of protein evolution is generally thought to reflect, at least in part, the proportion of amino acids within the protein that are needed for proper function. In the case of ADP-glucose pyrophosphorylase (AGPase), this premise led to the hypothesis that, because the AGPase small subunit is more conserved compared with the large subunit, a higher proportion of the amino acids of the small subunit are required for enzyme activity compared with the large subunit. Evolutionary analysis indicates that the AGPase small subunit has been subject to more intense purifying selection than the large subunit in the angiosperms. However, random mutagenesis and expression of the maize (Zea mays) endosperm AGPase in bacteria show that the two AGPase subunits are equally predisposed to enzyme activity-altering amino acid changes when expressed in one environment with a single complementary subunit. As an alternative hypothesis, we suggest that the small subunit exhibits more evolutionary constraints in planta than does the large subunit because it is less tissue specific and thus must form functional enzyme complexes with different large subunits. Independent approaches provide data consistent with this alternative hypothesis.

Isolation and characterization of cDNAs and genomic DNAs encoding ADP-glucose pyrophosphorylase large and small subunits from sweet potato

“…These observations confirm the idea that different AGPases in a given tissue have distinct regulatory properties depending on the specific LSs present (Ballicora et al 2004). Plant AGPase LS genes could be categorized into five major groups according to their sequences and expression patterns (Georgelis et al 2007). Group 1 comprised genes expressed in monocot and eudicot leaf tissue.…”

“…These data are in accord with the idea that tissue identity has influenced the rate of evolution of both SSs and LSs of AGPase. Expression in multiple tissues would then compound the various selection pressures exerted by the different tissues (Georgelis et al 2007). …”

“…In light of the high degree of sequence similarity, higher plant AGPase SS-and LSencoding genes are thought to have evolved from a common ancestor through gene duplication followed by divergence. It has been proposed that SSs in angiosperms have been subjected to a stricter evolutionary constraint than have LSs, as the former are less tissue-specific and must interact with different LSs (Georgelis et al 2007).…”

Characterization of an Autonomously Activated Plant ADP-Glucose Pyrophosphorylase

“…Recent studies by Georgelis et al (2007) showed that, whereas this difference is due to greater evolutionary constraints on the small subunit, the two subunits are equally predisposed to activity-altering mutations when expressed in one tissue with one complementary subunit. The difference in the rate of sequence divergence apparently is caused by the fact that plants generally have fewer small subunit genes than large subunit genes.…”

Heat Stability and Allosteric Properties of the Maize Endosperm ADP-Glucose Pyrophosphorylase Are Intimately Intertwined

“…While bacterial AGPases are homotetramers, plant and unicellular green algae AGPases are composed of two identical small subunits and two identical large subunits (for review, see Georgelis et al, 2007). Prokaryotic and eukaryotic AGPases have different quaternary structures and regulatory properties (Ballicora et al, 2003), yet their overall kinetic mechanisms appear to be similar (Paule and Preiss, 1971;Kleczkowski et al, 1993).…”