SAP54

Overview

Phytoplasmas are pathogenic bacteria that are obligate parasites of plants and transmitting insects [1-3]. They can cause devastating plant diseases, e.g. by reprogramming development in a way such that leaf-like structures instead of floral organs occur. Infected plants are thus often sterile, mainly serve to reproduce phytoplasmas and hence have been termed ‘zombie plants' [1, 3]. The molecular mechanism underlying the developmental reprogramming relies on specific interactions of a secreted phytoplasma protein called SAP54 (or PHYLLOGEN1) with a subset of MIKC-type MADS-domain transcription factors involved in controlling flower development [1, 2]. The secreted part of SAP54 interacts with the keratin-like domain (K domain) of MIKC proteins and destines them for degradation [1], so that they cannot constitute ‘floral quartets’ anymore, the protein complexes that specify floral organ identity [4]. Based on the recently published X-ray crystal structure of a K domain and detailed in silico analyses [5, 6] we have developed three hypotheses:
i) SAP54-like proteins form a structure which is very similar to that of the K domain;
ii) The interaction between SAP54 and the plant MIKC proteins is mediated by a mechanism that resembles the interaction of two K domains in floral quartets; 
iii) SAP54 mimics the K-domain structure as a result of convergent protein evolution [6]. 
We want to test these hypotheses by determining the structure of SAP54-like proteins and studying the interaction between these proteins and MIKC proteins.