Enzyme Is the Name – Adapter Is the Game

In bacteria, enzymes usually comprise of enzymatic domains only. In eukaryotes/mammals, and here particularly in context of signal transduction, proteins with enzymatic functions contain additional modules promoting interactions with cooperating signaling proteins. Often such protein-protein interaction domains, such as SH2- and SH3-domains, help to localize the enzymatic function (e.g. kinase, phosphatase) to another protein, cellular compartment, or membrane nanodomain. In such cases, the interaction domain(s) serve(s) the enzymatic function. However, there are molecular situations in which an interaction domain fulfills its task completely independent of the protein´s enzymatic function and in this review, we call such function the adapter function of an enzyme. Though many stories can be told, we will concentrate on several proteins playing crucial roles in cells of the immune system, such as BTK, PI3K, and SHIP1, as well as in cancer cells, such as proteins of the RAS/ERK MAPK pathway. We will also discuss how these adaptor functions of enzymes determine or even undermine the efficacy of targeted therapy compounds, such as ATP-competitive kinase inhibitors, thereby highlighting the need to develop pharmacological approaches, such as PROTACs, that eliminate the entire protein. We also review how genetic kock-out and knock-in approaches can be leveraged to identify adaptor functions of signaling proteins.