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Acidic ribosomal P proteins form a distinct lateral protuberance on the 60S ribosomal subunit. In yeast, this structure is composed of two heterocomplexes (P1A−P2B and P1B−P2A) attached to the ribosome with the aid of the P0 protein. In solution, the isolated P proteins P1A and P2B have a flexible structure with some characteristics of a molten globule [Zurdo, J., et al. (2000) Biochemistry 39, 8935−8943]. In this report, the structure of P1A−P2B heterocomplex from Saccharomyces cerevisiae is investigated by means of size-exclusion chromatography, chemical cross-linking, circular dichroism, light scattering, and fluorescence spectroscopy. The circular dichroism experiment shows that the complex could be ranked in the tertiary class of all-α proteins, with an average α-helical content of ∼65%. Heat and urea denaturation experiments reveal that the P1A−P2B complex, unlike the isolated proteins, has a full cooperative transition which can be fitted into a two-state folding−unfolding model. The behavior of the complex in the presence of 2,2,2-trifluoroethanol also resembles a two-state folding−unfolding transition, further supporting the idea that the heterocomplex contains well-packed side chains. In conclusion, the P1A−P2B heterocomplex, unlike the isolated proteins, has a well-defined hydrophobic core. Consequently, the complex can put up its structure without additional ribosomal components, so the heterodimeric complex reflects the intrinsic properties of the two analyzed proteins, indicating thus that this is the only possible configuration of the P1A and P2B proteins on the ribosomal stalk structure.