Identification of LACTB2, a metallo-ß-lactamase protein, as a human mitochondrial endoribonuclease.

Post-transcriptional control of mitochondrial gene expression, including the processing and generation of mature transcripts as well as their degradation, is a key regulatory step in gene expression in human mitochondria. Consequently, identification of the proteins responsible for RNA processing and degradation in this organelle is of great importance. The metallo-ß-lactamase (MBL) is a candidate protein family that includes ribo- and deoxyribonucleases. In this study, we discovered a function for LACTB2, an orphan MBL protein found in mammalian mitochondria. Solving its crystal structure revealed almost perfect alignment of the MBL domain with CPSF73, as well as to other ribonucleases of the MBL superfamily. Recombinant human LACTB2 displayed robust endoribonuclease activity on ssRNA with a preference for cleavage after purine-pyrimidine sequences. Mutational analysis identified an extended RNA-binding site. Knockdown of LACTB2 in cultured cells caused a moderate but significant accumulation of many mitochondrial transcripts, and its overexpression led to the opposite effect. Furthermore, manipulation of LACTB2 expression resulted in cellular morphological deformation and cell death. Together, this study discovered that LACTB2 is an endoribonuclease that is involved in the turnover of mitochondrial RNA, and is essential for mitochondrial function in human cells.

ABL1 in thalamus is associated with safety but not fear learning.

In auditory fear conditioning a tone is paired with a footshock, establishing long lasting fear memory to the tone. In safety learning these stimuli are presented in an unpaired non-overlapping manner and enduring memories to the tone as a safety signal are formed. Although these paradigms utilize the same sensory stimuli different memories are formed leading to distinct behavioral outcome. In this study we aimed to explore whether fear conditioning and safety learning lead to different molecular changes in thalamic area that receives tone and shock inputs. Toward that end, we used antibody microarrays to detect changes in proteins levels in this brain region. The levels of ABL1, Bog, IL1B, and Tau proteins in thalamus were found to be lower in the group trained for safety learning compared to the fear conditioning group 6 h after training. The levels of these proteins were not different between safety learning and fear conditioning trained groups in auditory cortex. Western blot analysis revealed that the ABL1 protein level in thalamus is reduced specifically by safety learning but not fear conditioning when compared to naïve rats. These results show that safety learning leads to activation of auditory thalamus differently from fear conditioning and to a decrease in the level of ABL1 protein in this brain region. Reduction in ABL1 level in thalamus may affect neuronal processes, such as morphogenesis and synaptic efficacy shown to be intimately regulated by changes in this kinase level.