What is the meaning of the word "Ligandable" in "ligandable proteome"?
In biochemistry, ligand is usually taken to mean a substance which binds to a larger molecule such as an enzyme or other protein, thereby forming a larger complex.  The study of ligand binding has been important for pharmaceuticals, particularly in ligand-based drug design.

The term ligandability (and hence ligandable) seems to have come into significant use with a paper by Edfeldt, Folmer, and Breeze in which they say:


  In recent years, the term druggability has increasingly been adopted to describe the ability of a protein target to bind small molecules with high affinity; however, we believe the term ‘ligandability’ is more appropriate for this purpose. Although ligandability is a requirement for finding drugs for a particular target, it is not a guarantee that such ligands will make good drugs.


So, ligandable usually refers to the capability of a molecular target of drug design to bind small molecules.  A ligandable proteome would then be a set of proteins capable of binding small molecules.