  Apart from the knots formed by the main-chain, the proteins can form numerous
topological structures, when included the covalent and ion-mediated
interactions. In this work, we define the protein non-trivial $\theta$-curves
and identify 7 different topologies in all structures known up to date. We
study the correlation of the motif with the function and organism of origin,
and pointing the similarity with main-chain knots, we show that some motifs may
indeed be functional. We also analyze the folding and bridge-induced stability
of an exemplary protein with $\theta$-curve motif and provide a catalogue of
possible $\theta$-curves in proteins.
