In vivo protein synthesis is carried out by the ribosome which is a complex of proteins and RNA. The ribosomal proteins, although essential for the correct function of the ribosome, provide only structural support to hold the rRNA in the correct conformation; the RNA acts as the catalyst for protein synthesis in the ribosome.
However, the ribosome can be denatured at high temperatures, since RNA secondary structure is succeptible to thermal denaturation. In addition, heat will denature proteins upstream of translation such as RNA polymerase II, and the proteins involved in capping and polyadenylation (which are essential for producing the mRNA template for protein synthesis).