Involved in phycobilisome (PBS) degradation during nutrient deprivation. May mark the PBS for degradation by covalent association with PBS components or may disrupt the PBS via ionic interactions. Present at very low levels in nutrient-replete cells, and at 50-fold higher levels in nitrogen- or sulfur-deprived cells and to a variable extent (<10-fold) during phosphorus deprivation. To chloroplast ycf18.