TPK1, TPK2, and TPK3 encode isoforms of the catalytic subunit of cAMP-dependent protein kinase (PKA), the effector kinase of the Ras-cAMP signaling pathway (reviews of the Ras-cAMP pathway can be found in 7 and 8). Through phosphorylation of various targets, PKA activity regulates processes involved in cell growth and response to nutrients and stress, such as nutrient sensing, energy metabolism, carbohydrate utilzation, cell cycle progression, thermotolerance, osmotic shock tolerance, sporulation, bud site selection, pseudohyphal growth, aging, and autophagy (reviewed in 8, 9, 10, and 11). PKA substrates include transcription factors (e.g., Rap1p (12), Hsf1p (13), Adr1p (14), Msn2p / Msn4p (15, 16), Ssn2p (17)), metabolic enzymes (e.g., Cho1p (18), Pfk2p (19), Nth1p (20)), and other regulatory kinases (e.g., Rim15p (21), Atg1p (22)). The PKA target site is R-[KR]-x-S, where S is the site of substrate phosphorylation (23, 24). PKA is conserved from yeast to man and TPK homologs have been identified in fission yeast, flies, worms, mice, pigs, cows, and humans (25, and reviewed in 26).

In the absence of cAMP, the catalytic subunits (Tpk1p, 2p, or 3p) form an inactive heterotetrameric complex with the PKA regulatory subunit Bcy1p, with two catalytic subunits bound to two regulatory subunits. In the presence of cAMP, Bcy1p binds to cAMP and dissociates from the complex as a homodimer, releasing the two catalytic subunits as active monomers (2 and references therein). PKA is also regulated through a cAMP feedback inhibition loop: PKA activity leads to a reduction in cAMP levels by directly phosphorylating enzymes which regulate cAMP production, (e.g. the GTPase Ras2p and the cAMP phosphodiesterase Pde1p), and the resulting decrease in cAMP levels promotes formation of the inactive heterotetrameric complex (27, 28, 29, 30). All three Tpk isozymes are phosphorylated and the phosphorylation state of Tpk1p has been shown to affect its substrate specificity constant and to be correlated with the availability of a fermentable carbon source (31).

The three PKA catalytic subunits are highly similar only in their C-terminal regions. Although the C-terminal 320 amino acids of Tpk1p and Tpk2p share 77% identity and are 88% and 75% identical to Tpk3p respectively, the N-terminal regions of these proteins share no sequence similarity and are heterogeneous in length (2). While no single TPK gene is essential, the presence of at least one of the isomers is required for normal growth (2). Mutations affecting levels of PKA activity lead to pleiotropic phenotypes because of the wide range of PKA substrates and their involvement in many cellular processes. Phenotypes due to hyperactive PKA signaling include rapamycin resistance, accelerated pseudohyphal growth, transient cell cycle arrest in G1, and increased sensitivity to stresses such as heat shock and nutrient starvation while some of the phenotypes resulting from reduced PKA activity are hyperaccumulation of cAMP, defective glucose repression, and decreased levels of ribosomal protein mRNAs (32, 33 and references therein, and reviewed in 7).

Although the Tpk isomers are functionally redundant for cell viability, they appear to regulate different processes. One high-throughput study showed that different numbers of distinct proteins were targeted by Tpk1p (256), Tpk2p (29), and Tpk3p (79) and greater than 87% of all PKA substrates were uniquely phosphorylated by just one of the Tpks (23). In addition to protein targets, the three isomers also recognize and affect the transcription of different sets of gene targets. Tpk2p was shown to bind to the promoter region of genes encoding ribosomal proteins and in particular regulates genes involved in iron uptake, trehalose degradation, and water homeostasis (34, 35). Tpk2p also inhibits the function of the anaphase-promoting complex (APC) through possible phosphorylation of the APC activator Cdc20p (36, 37).

Last updated: 2007-05-17