**3. Conclusion**

Combination of affinity based cross-linking of proteins with specific DNA probes containing intact or cleaved AP sites and MS analysis allowed to identify new players in recognition/processing of these ubiquitous lesions. Some identified proteins are known as regulatory proteins of specific DNA repair processes, not necessarily involved in repair of AP sites, but all these proteins are related to cell radiosensitivity. Two of them—XRCC1 and PARP1—belong to the base excision repair system, but have been previously considered as participants of later stages of the BER process. By virtue of its interaction with AP sites, PARP1 and XRCC1 can regulate initial stage of BER persisting at AP sites until APE1 could come and initiate AP sites cleavage. The PARP1 activation upon interaction with AP sites and resultant automodification appear to facilitate the BER factor recruitment to stimulate the repair process. In the absence or deficiency of APE1, PARP1 can provide temporal protection of AP sites. Ku antigen can temporarily protect AP sites situated within 1-2 turns of DNA helix from double-strand breaks or displays AP/dRP lyase activity near doublestrand ends. The last activity has no analogues and is indispensable for proper repair of DS breaks with particular blocking groups at the ends, which can result from action of ionizing radiation.

In other case of revealed AP/dRP lyase activities, they appear to function under particular physiologic or stressful conditions modulating the capacity of the BER system or providing the appropriate functions of the repair machinery when some participants are missed or inactive. Interestingly, that structural chromatin proteins –HMGB1 and HMGA – were found to interact with AP sites displaying an enzymatic activity. Although being a relative weak, this activity may be biologically significant due to high abundance of these proteins.
