The caliphate was claimed by the sultans of the Ottoman Empire beginning with Murad I (reigned 1362 to 1389), while recognising no authority on the part of the Abbasid caliphs of the Mamluk-ruled Cairo. Hence the seat of the caliphate moved to the Ottoman capital of Edirne. In 1453, after Mehmed the Conqueror's conquest of Constantinople, the seat of the Ottomans moved to Constantinople, present-day Istanbul. In 1517, the Ottoman sultan Selim I defeated and annexed the Mamluk Sultanate of Cairo into his empire. Through conquering and unifying Muslim lands, Selim I became the defender of the Holy Cities of Mecca and Medina, which further strengthened the Ottoman claim to the caliphate in the Muslim world. Ottomans gradually came to be viewed as the "de facto" leaders and representatives of the Islamic world. However, the earlier Ottoman caliphs did not officially bear the title of caliph in their documents of state, inscriptions, or coinage. It was only in the late eighteenth century that the claim to the caliphate was discovered by the sultans to have a practical use, since it allowed them to counter Russian claims to protect Ottoman Christians with their own claim to protect Muslims under Russian rule.